Russian warplanes violated Ukraine's airspace several times in the past 24 hours, a Pentagon spokesman said Friday, in the latest sign of a mounting confrontation between Moscow and Kiev.

"I can confirm that on several occasions in the last 24 hours, Russian aircraft have entered Ukrainian airspace," said Colonel Steven Warren.

Latvia on Friday welcomed American troops on its soil, part of a U.S. force of 600 sent to the region to reassure the Baltic states amid concern over Russia's actions in Ukraine.

"Today is a special day because this morning I met the heads of the armed forces at the Adazi military base and greeted the U.S. military unit that arrived this morning for military training," Latvian Prime Minister Laimdota Straujuma told reporters.

Western powers led by U.S. President Barack Obama warned Friday that Russia faced fresh sanctions over Ukraine as Kiev accused Moscow of seeking to trigger a "third world war".

The threat came amid heightened tensions on the ground as the Ukrainian military launched a new offensive to besiege the rebel-held city of Slavyansk and insurgents blew up an army helicopter with a rocket-propelled grenade.

Kiev accused Moscow on Friday of seeking to trigger a "third world war" as military tensions soared in east Ukraine and U.S. President Barack Obama led a diplomatic charge against Russia.

A rocket-propelled grenade blew up a Ukrainian military helicopter sitting on the tarmac at a base near the eastern town of Kramatorsk, officials in Kiev said. The pilot escaped but was wounded. Smoke spewed in the sky over the nearby rebel-held town.

Russia Foreign Minister Sergei Lavrov on Friday accused the United States of seeking to smear Moscow and called U.S. Secretary of State John Kerry's tone over the Ukraine crisis unacceptable.

Lavrov warned "the might of U.S. propaganda" was aimed "at smearing Russia, smearing those who protest against the illegal actions of the (Kiev) authorities who are trying to ban Russian and trying to call all Russians and Russian speakers enemies who should be killed."

The United States warned Russia Thursday against making an "expensive mistake" in Ukraine, as Moscow ordered new military exercises on its shared border after Kiev forces attacked pro-Kremlin rebels.

U.S. Secretary of State John Kerry warned that Russia's refusal to take any steps to end the crisis in Ukraine would prove costly, saying the window for Moscow to change course was closing.

Ukraine's military launched assaults to retake rebel-held eastern towns on Thursday in which up to five people were reported killed, a move Russian President Vladimir Putin warned would have "consequences."

The offensive sent international tensions soaring and oil prices up on the prospect of Russia making good on its threat of a massive response in the ex-Soviet republic.

President Francois Hollande will visit Georgia shortly, his foreign minister announced Thursday as France seeks to boost ties with the small ex-Soviet country that stands in the shadow of its giant neighbor Russia.

The French leader's visit -- due in May -- comes at a time when the European Union is seeking to boost ties with countries lying to its east, and as the West and Russia exchange increasingly strident barbs over the crisis in Ukraine where Moscow has already annexed the Crimean peninsula.

Russian Foreign Minister Sergei Lavrov on Thursday accused the United States and the European Union of being behind a popular uprising in Ukraine that ousted pro-Kremlin president Viktor Yanukovych in February.

"In Ukraine the United States and the European Union tried to stage -- let's call things what they are -- another 'color revolution', an operation to unconstitutionally change regime," the Interfax news agency quoted Lavrov as saying.

President Barack Obama said Thursday Russia was not abiding by the letter or the spirit of the Geneva deal to ease tensions in Ukraine, and said new sanctions against Moscow were already teed up.

The president weighed into an increasingly bitter U.S.-Russian dispute over the actions of separatist militias in the east of the country Washington says are backed by the Kremlin, during his tour of Asia.