A Ukrainian border patrol plane came under fire while flying at about 1,000 metres near the administrative border with Russian-occupied Crimea on Saturday, border guards said as the crisis in the region deepened.

No one was hurt when gunmen opened fire on the unarmed aircraft, a spokesman said. The Diamond light aircraft was flying three crew on an observation mission, the spokesman said.

Meanwhile, warning shots were fired to prevent an unarmed international military observer mission from entering Ukraine's Russian-occupied Crimea on Saturday.

U.S. Secretary of State John Kerry warned Russia on Saturday that any steps to annex Ukraine's Crimea region would close the door to diplomacy, a U.S. State Department official said.

Kerry's latest telephone call with his Russian counterpart, Foreign Minister Sergei Lavrov, came as the standoff between occupying Russian forces and besieged Ukrainian troops intensified in Crimea.

"He made clear that continued military escalation and provocation in Crimea or elsewhere in Ukraine, along with steps to annex Crimea to Russia would close any available space for diplomacy, and he urged utmost restraint," the official said.

U.S. President Barack Obama made a series of phone calls on Saturday to world leaders about the ongoing crisis, including British Prime Minister David Cameron, French President Francois Hollande, and Italian Prime Minister Matteo Renzi the White House said.

He also held a conference call about the situation with the presidents of Lithuania, Latvia and Estonia. 

Russia's seizure of the Black Sea peninsula, which began about 10 days ago, has so far been bloodless, but its forces have become increasingly aggressive towards Ukrainian troops, who are trapped in bases and have offered no resistance.

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People hold a banner reading, "We demand a referendum" as they shout slogans during a pro-Russian rally in Donetsk, Ukraine, on Saturday. (Sergei Grits/Associated Press)

Tempers have grown hotter in the last two days, since the region's pro-Moscow leadership declared it part of Russia and announced a March 16 referendum to confirm it.

A spokeswoman for the Organization for Security and Cooperation in Europe said no one was hurt when shots were fired to turn back its mission of more than 40 unarmed observers, who have been invited by Kyiv but do not have permission from Crimea's pro-Russian separatist regional authorities.

They had been turned back twice before, but this was the first time shots were fired.

Kyiv's security council said it had been targeted by hackers in a "massive" denial of service attack designed to cripple its computers. The national news agency was also hit, it said.

President Vladimir Putin declared a week ago that Russia has the right to invade Ukraine to protect Russian citizens, and his parliament has voted to change the law to make it easier to annex territory.

Ukraine troops trapped

Russian troops drove a truck into a missile defence post in Sevastopol, the home of both their Black Sea Fleet and the Ukrainian navy, and took control of it overnight. A Reuters reporting team at the scene said no-one was hurt.

'The situation is changed. Tensions are much higher now. You have to go. You can't film here.'- Russian soldier to media

Ukraine's border service said Russian troops had also seized a border guard outpost in the east of the peninsula overnight, kicking the Ukrainian officers and their families out of their apartments in the middle of the night.

"The situation is changed. Tensions are much higher now. You have to go. You can't film here," said a Russian soldier carrying a heavy machine gun, his face covered except for his eyes, at a Ukrainian navy base in Novozernoye.

Polish Foreign Minister Radoslaw Sikorski said on Saturday Poland had evacuated its consulate in Sevastopol due to "continuing disturbances by Russian forces."

About 100 armed Russians are keeping watch over the Ukrainians at the base, where a Russian ship has been scuttled at the entrance to keep the Ukrainians from sailing out.

"Things are difficult and the atmosphere has got worse. The Russians threaten us when we go and get food supplies and point their guns at us," said Vadim Filipenko, the Ukrainian deputy commander at the base.

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Volunteers of a self defence group lean on their shields at the swearing in of the first unit of a pro-Russian armed force in Simferopol. (Vadim Ghirda/Associated Press)

The situation is particularly tense for those of Tatar origin in Crimea. A minority ethnic group indigenous to the region, the Tatars have a history of strained relations with Russians. They were expelled from Crimea by Joseph Stalin after the Second World War and only allowed to return in the 1960s.

Khalil Khalilov, a Tatar who moved to Canada in 2004, still has many friends and family in Crimea.

Khalilov says some homes in the region have been marked with an "X" to indicate the people living there are Tatars or from other groups. Even with rising tensions, Tatars will likely stick it out, he told CBC News on Saturday.

"Tatars were always desperate to come back to Crimea and they're definitely not going to give up. So they are very firm in their feelings...and to not leave their home place."

Many hotheads

Moscow denies that the Russian-speaking troops in Crimea are under its command, an assertion Washington dismisses as "Putin's fiction". Although they wear no insignia, the troops drive vehicles with Russian military plates and identify themselves as Russian troops to the besieged Ukrainian forces.

The United States has announced sanctions against individuals it blames for interfering with Ukrainian territorial integrity, although it has yet to publish the list. The European Union is also considering sanctions, although this may be much harder to organize for a 28-member bloc that must take decisions unanimously and depends on Russian natural gas.

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Pro-Russian activists continued to gather on Saturday in the eastern Ukrainian city of Donetsk, as Russia was reported to be reinforcing its military presence in Crimea. (Sergei Chuzavkov/Associated Press)

Russian Foreign Minister Sergei Lavrov gave no indication of any softening of Moscow's position on Saturday, insisting that the government in Kyiv had been installed in an illegal coup.

Crimea's pro-Moscow leader Sergei Aksyonov said the referendum on union with Russia — now due in a week — had been called so quickly to avert "provocation."

"There are many hotheads who are trying to create a destabilized situation in the autonomous republic of Crimea, and because the life and safety of our citizens is the most valuable thing, we have decided to curtail the duration of the referendum and hold it as soon as possible," he told Russian television.

Aksyonov, whose openly separatist Russian Unity political received just 4 percent of the vote in Crimea's last parliamentary election, declared himself provincial leader ten days ago after armed Russians seized the parliament building.

Independence vote in 1991

Crimean opposition parliamentarians say most lawmakers were barred from the besieged building, both for the vote that installed Aksyonov and the one a week later that declared Crimea part of Russia, and the results were falsified. Both votes took place behind closed doors.

Crimea has a narrow ethnic Russian majority, but it is far from clear that most residents want to be ruled from Moscow. When they were last asked in 1991, they voted for independence along with the rest of Ukraine. Western countries have dismissed the upcoming referendum as illegal and likely to be falsified.

'With all these soldiers here, it is like we are living in a zoo. Everyone fully understands this is an occupation.'-  Tatyana, 41, an ethnic Russian

Many in the region do feel deep hostility to Kyiv, and since Aksyonov took power supporters of union with Moscow have controlled the streets, waving Russian flags and chanting "Rossiya! Rossiya!"

Nevertheless, many still quietly speak of their alarm at the Russian takeover.

"With all these soldiers here, it is like we are living in a zoo," Tatyana, 41, an ethnic Russian. "Everyone fully understands this is an occupation."

The region's 2 million population includes 250,000 indigenous Tatars, who only returned since the 1980s after being deported en masse to distant Uzbekistan by Stalin. They are fiercely opposed to Russian annexation.

The referendum is "completely illegitimate. It has no legal basis," Crimean Tatar leader Refat Chubarev told Germany's Suddeutsche Zeitung. "Different groups of people with different histories live here. A mathematical majority cannot express the wishes of the population."

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A memorial commemorates those who died in clashes which took place in late February, in Kyiv's Independence Square on Saturday. (David Azia/Associated Press)

Journalists have been attacked by hostile crowds. The Associated Press said armed men had confiscated TV equipment from one of its crews.

In addition to the Russian troops, the province is prowled by roving bands of "self defence" forces and Cossacks in fur hats armed with whips who were bused in from southern Russia.

Russian television and the provincial channel controlled by Aksyonov broadcast relentless inaccurate accounts of "fascists" in control of the streets in Kyiv and of plans to ban the Russian language. Ukrainian television and the region's only independent station have been switched off.

Gazprom arrears

Ukraine slid into its confrontation with Moscow within days of pro-Russian President Viktor Yanukovych's flight from the country. Yanukovych was toppled after three months of demonstrations against a decision to spurn a free trade deal with the European Union for closer ties with Russia.

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A uniformed man, believed to be a Russian serviceman, stands guard near a Ukrainian military base outside the city of Sevastopol in Crimea. (David Mdzinarishvili/Reuters)

The final week of protests saw around 100 people were killed in the streets, many shot by sharpshooters on rooftops.

The European Commission has said Ukraine could receive up to €11 billion ($15 billion USD) in the next couple of years provided it reaches agreement with the IMF, which requires economic reforms that Yanukovych had resisted.

The money, urgently needed to stave off bankruptcy, effectively replaces $15 billion offered to Yanukovych by Putin as a prize for spurning the EU trade deal.

Promises of billions of dollars in Western aid for the Kyiv government, and the perception that Russian troops are not likely to go beyond Crimea into other parts of Ukraine, have helped reverse a rout in the local hryvnia currency.

Russian gas monopoly Gazprom said Ukraine had not paid its $440 million gas bill for February, bringing its arrears to $1.89 billion, and hinted it could turn off the taps as it did in 2009, when a halt in Russian deliveries to Ukraine reduced supplies to Europe during a severe winter.

With files from CBC News