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 criticized what he called Russia's "further meddling" in eastern Ukraine, telling reporters in Seoul on Friday that he would talk to "key European leaders" later in the day about implementing wider sanctions in the event Russia further escalates the situation.
He said 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.