Ukrainian Prime Minister Arseniy Yatsenyuk is accusing Russia of wanting to occupy Ukraine "militarily and politically," as both Kyiv and Moscow mass troops close to their mutual border.
Mr. Yatsenyuk warned Friday that Russia's actions could lead to a wider military conflict in Europe. He told an interim Cabinet meeting that Moscow "wants to start World War Three."
His comments came amid reports of fresh violence in eastern Ukraine, where pro-Russian militants continue to occupy government buildings in around a dozen cities.
A Ukrainian military helicopter exploded Friday on the tarmac of a base near the eastern city of Kramatorsk. Some Ukrainian officials said the explosion was the result of a rocket-propelled grenade fired by pro-Russian militants, while others said it was caused by a sniper who fired a single shot into a fuel tank.
Ukrainian officials said Friday that they aimed to "blockade" the rebel-held city of Slovyansk as part of an "anti-terrorist" operation in eastern Ukraine. Officials said on Thursday that the crackdown had killed up to five people.
Russian Foreign Minister Sergei Lavrov on Friday denounced Kyiv's security operation to clear the pro-Russian militants, calling it a "bloody crime."
Meanwhile, U.S. President Barack Obama discussed the Ukraine crisis Friday in a telephone call with French President François Hollande, German Chancellor Angela Merkel, Italian Prime Minister Matteo Renzi and British Prime Minister David Cameron.
The White House said in a statement that the four leaders agreed Ukraine had taken "positive steps" to uphold the four-party deal it signed in Geneva last week to de-escalate tensions, while Russia had "not reciprocated."
According to the White House, Mr. Obama said the United States is prepared to impose targeted sanctions to respond to Russia's latest actions, and all four leaders agreed to closely coordinate additional steps to impose costs on Russia.
Speaking to reporters in Seoul earlier Friday, Mr. Obama criticized what he called Russia's "further meddling" in eastern Ukraine, saying Russian President Vladimir Putin must decide whether he wants to see his country's already fragile economy weakened further because he failed to act diplomatically in Ukraine.
Russia has reportedly dramatically increased the number of troops deployed along its border with Ukraine. A Ukrainian diplomat at the United Nations told VOA that Moscow has doubled its military presence on the border to about 80,000 troops.
Lavrov, for his part, blamed the West for raising tensions, saying Friday that the pro-Russian militants would only lay down their weapons if the Ukrainian government first clears out its own protesters in the capital.
Underscoring the effect that wider sanctions could have on Russia's economy, credit agency Standard and Poor's cut Russia's credit rating to BBB- . The agency said it is concerned about increased capital outflows from Russia, and said the rating could be cut further if sanctions are tightened.
Washington has accused Moscow of failing to uphold the four-party deal it signed last week calling for all parties in Ukraine to lay down their weapons and vacate public buildings. U.S. Secretary of State John Kerry said Moscow has not taken "a single step" to de-escalate tensions since the deal was signed in Geneva.