Canada will send up to 500 people to monitor Ukraine's presidential election next month, Prime Minister Stephen Harper said Wednesday.

The Canadian delegation is to include up to 338 people in an election observation mission.

"Canada is a world leader in helping to promote democracy and good governance in Ukraine," Harper said in a statement.

"The measures announced today will help ensure that the upcoming Ukrainian elections are free and fair and that democracy and governance are the cornerstones of the new government."

Another group of up to 150 observers and 12 parliamentarians will join a election-monitoring mission from the Organization for Security Co-operation in Europe.

The government has earmarked $11 million to cover the costs of the observers.

The election is scheduled for May 25 and comes amid growing concerns about separatist tensions in the eastern part of the country.

Harper said Canada wants to ensure that Ukrainians can freely choose a leader without coercion or intimidation.

Since 2004, Canada has sent hundreds of observers to five successive Ukrainian elections.

The prime minister has taken a keen interest in Ukraine even before the latest crisis, making clear his support for democratic institutions there.

Last month, he made a whirlwind visit to Ukraine, where he met acting president Oleksandr Turchynov and Prime Minister Arseniy Yatsenyuk.

Russia officially annexed Crimea last month and there are worries that Moscow may have its sights on more of the country.

Harper has taken a hard line against Russian President Vladimir Putin, saying his regime is a threat to world peace. Canada and Russia have also been engaged in a tit-for-tat exchange of expelled diplomats of late.

Russia vows to react to attacks in Ukraine

Also on Wednesday, Russia's foreign minister promised a firm response if its citizens or interests come under attack in Ukraine — a vow that came after Ukraine announced a renewal of its "anti-terror" campaign against those occupying buildings in its troubled east.

Although Foreign Minister Sergey Lavrov did not specifically say Russia would launch a military attack, his comments bolstered wide concern that Russia could use any violence in eastern Ukraine as a pretext for sending in troops. Large contingents of Russian troops — tens of thousands, NATO says — are in place near the Ukrainian border.

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Russian Foreign Minister Sergey Lavrov speaks during a news conference after the meeting of foreign ministers from Caspian countries, in Moscow, Russia, on Tuesday, April 22, 2014. (AP Photo/Ivan Sekretarev) (Ivan Sekretarev/Associated Press)

"Russian citizens being attacked is an attack against the Russian Federation," Lavrov said in an interview with Kremlin-funded satellite TV channel RT. "If our interests, our legitimate interests, the interests of Russians have been attacked directly, like they were in South Ossetia, I do not see any other way but to respond in full accordance with international law."

It was unclear from the interview what Russia would regard as its interests in Ukraine. President Vladimir Putin previously has said Russia would be justified in protecting ethnic Russians in Ukraine.

Ukraine's acting president on Tuesday ordered resumption of an "anti-terrorist operation" against pro-Russia forces that have seized police stations and government in at least 10 cities and towns in eastern Ukraine. The order came after the bodies of two people allegedly abducted by pro-Russia insurgents were found.

However, there were no reports Wednesday of any actions taken by the Ukrainian military or security services.

U.S. journalist being held by pro-Russia forces

Pro-Russia forces admitted Wednesday they are holding an American journalist, saying he was suspected of spying for Ukrainian ultra-nationalists.

Simon Ostrovsky, a journalist for the Brooklyn-based Vice News, has not been seen since early Tuesday in the eastern city of Slovyansk. The fluent Russian-speaker who also holds an Israeli passport has been covering the crisis in Ukraine for weeks and was reporting about the groups of masked gunmen seizing government buildings in one eastern Ukrainian city after another.

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Masked pro-Russia protestors guard barricades at the regional administration building that they had seized earlier in Donetsk, eastern Ukraine, Tuesday. (Sergei Grits/Associated Press)

The insurgents in the east are defying last week's international agreement in Geneva that called for all sides to disarm militant groups in Ukraine and to vacate public buildings they are occupying.

Members of the nationalist Right Sector movement have occupied two buildings in the capital, Kyiv, for months, but Ukraine authorities have said the priority is to get the gunmen in eastern Ukraine to vacate the government buildings they hold.

U.S. Vice President Joe Biden visited Kyiv, the capital, on Tuesday to offer support to the beleaguered interim government.

Since November, Ukraine has been engulfed in its biggest political crisis since the 1991 fall of the Soviet Union. Months of anti-government protests in Kiev culminated in President Viktor Yanukovych fleeing to Russia in late February.

Ukraine's acting government has accused Russia of orchestrating the unrest in eastern Ukraine, which it fears Moscow could use as a pretext for an invasion. Last month, Russia annexed Crimea weeks after seizing control of the Black Sea peninsula.