Indiscriminate rocket fire slammed into a market, schools, homes and shops Saturday in Ukraine's southeastern city of Mariupol, killing at least 30 people, authorities said. The Ukrainian president called the blitz a terrorist attack and NATO and the U.S. demanded that Russia stop supporting the rebels.

Ukrainian officials rushed to defend the strategically important port on the Sea of Azov, beefing up military positions with more equipment and sending in more forces.

The separatists' top leader declared that an offensive against Mariupol had begun — then later toned down his threats as the scale of the civilian casualties became clear.

Ukrainian President Petro Poroshenko held an emergency meeting of his military officials and cut short a trip to Saudi Arabia to co-ordinate the government's response.

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Shelling by pro-Russian rebels in a residential sector of Mariupol in eastern Ukraine on Saturday has killed at least 30 people, authorities said. (Nikolai Ryabchenko/Reuters)

"The time has come to name their sponsors. The help given to militants, weapons deliveries, equipment and the training of manpower — is this not aiding terrorism?" Poroshenko said in a recorded statement.

Russia insists it does not support the rebels, but Western military officials say the sheer number of heavy weapons under rebel control belies that claim.

An AP reporter saw convoys of pristine heavy weapons in rebel territory earlier this week.

Fresh offensive

The rocket attacks came a day after the rebels rejected a peace deal and announced they were going on a multi-prong offensive against the government in Kyiv to vastly increase their territory. The rebel stance has upended European attempts to mediate an end to the fighting in eastern Ukraine, which the UN says has killed nearly 5,100 people since April.

'Russian troops in eastern Ukraine are supporting these offensive operations with command-and-control systems, air defence systems with advanced surface-to-air missiles, unmanned aerial systems, advanced multiple rocket launcher systems, and electronic warfare systems' - NATO Secretary General Jens Stoltenberg

Mariupol, a major city under government control, lies between mainland Russia and the Russia-annexed Crimean Peninsula. Heavy fighting in the region in the fall raised fears that Russian-backed separatist forces would try to capture city to establish a land link between Russia and Crimea.

Ukraine's defence ministry said three separate strikes from Grad multiple-rocket launchers hit Mariupol and surrounding areas Saturday.

"The area that came under attack was massive," Mariupol mayor Yuriy Khotlubei said. "The shelling was carried out by militants. This is very clearly Russian aggression that has caused terrible losses for the residents of the eastern part of our city."

The Donetsk regional government loyal to Kyiv said at least 30 people — including a 15-year old girl and a five-year old boy — have died in the attacks. A Ukrainian military checkpoint near the city was also hit and one serviceman was killed, the defence ministry said.

Increasing violence

The RIA Novosti news agency cited Ukrainian rebel leader Alexander Zakharchenko as saying an offensive had begun on Mariupol. He spoke as he laid a wreath Saturday where at least eight civilians died when a bus stop was shelled Thursday in Donetsk, the largest rebel-held city in eastern Ukraine.

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Ukraine's interior ministry blamed the attack on the port city on separatist rebels and the Russian military. (Reuters)

Zakharchenko swiftly backtracked, however. He denied that his forces were responsible for Saturday's carnage, saying it was caused by Ukrainian error. He also said the Ukrainian defensive positions around Mariupol would be destroyed but the city itself would not be stormed.

But the Organization for Security and Cooperation's monitoring mission said the attack in Mariupol was caused by Grad and Uragan rockets fired from areas under rebel control.

Rebel forces have positions 10 kilometres from Mariupol's eastern outskirts. On Jan. 13, a bus near an army checkpoint north of Mariupol was hit by a shell, killing 13 people, an attack Ukraine blamed on the separatists.

Reinforcements were being drafted into the city and the Mariupol-based Azov Battalion was being equipped with more heavy weaponry, Interior Minister Arsen Avakov said on Facebook. Security services also detained a spotter suspected of giving rebel fighters coordinates to launch rockets, he said.

Russia's 'dangerous' decision

Prime Minister Arseniy Yatsenyuk on Friday had ordered regional leaders to draw up economic blueprints to put the country on a war footing. Ukraine began its fourth wave of mobilization this week, building up manpower for its faltering war effort.

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People set candles on Independence Square in Kyiv in memory of people who died during shelling in Mariupol. (Anatolii Stepanov/AFP/Getty Images)

U.S. Secretary of State John Kerry said the rebels' new offensive "has been aided and abetted by Russia's irresponsible and dangerous decision to resupply them in recent weeks with hundreds of new pieces of advanced weaponry."

"I join my European counterparts in condemning in the strongest terms today's horrific assault by Russia-backed separatists on civilian neighbourhoods in Mariupol," Kerry said in a statement, citing reports of dozens wounded as well.

He urged Russia to end its support for separatists, close its international border with Ukraine and withdraw all weapons, fighters and financial backing or face increased U.S. and international pressure.

Restarting the peace process

NATO Secretary General Jens Stoltenberg also condemned the Mariupol shelling and what he said was the increased presence of Russian forces in Ukraine.

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A Ukrainian serviceman stands guard as emergency workers inspect a damaged bridge. An overnight blast destroyed a railway bridge in the region, cutting a railway link between Mariupol and Ukraine's west. (Reuters)

"Russian troops in eastern Ukraine are supporting these offensive operations with command-and-control systems, air defence systems with advanced surface-to-air missiles, unmanned aerial systems, advanced multiple rocket launcher systems, and electronic warfare systems," he said.

A peace deal signed in September in the Belarusian capital of Minsk envisaged a cease-fire and a pullout of heavy weapons from a division line in eastern Ukraine, but that was repeatedly violated by both sides. Foreign ministers from Russia, Ukraine, France and Germany agreed Wednesday to revive that division line but the rebels on Friday rejected the whole Minsk deal.

Senior envoys from Ukraine, Russia and the OSCE issued a statement Saturday convening an urgent meeting next week to restart the Minsk peace process.