Russia on Friday said it had proof the Syrian government was not behind a new spate of alleged chemical attacks in the country, amid fresh international calls for a probe.

"Accusations against government forces continue to be fabricated about alleged cases of them using chemical substances," said a Russian foreign ministry statement.

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.

Cambodia's highest court on Friday rejected a request by Russia to extradite a tycoon accused of embezzling tens of millions of dollars in a real estate scam.

The Supreme Court ruled that Sergei Polonsky, 41, could not be sent back to Russia because the two countries do not have an extradition treaty.

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.

Russian opposition leader Alexei Navalny escaped Thursday the imminent threat of jail when a court extended his arrest, but did not put the top critic of President Vladimir Putin in pre-trial detention.

The 37-year-old anti-corruption blogger, who has been under house arrest since February and banned from using the Internet, saw his detention extended until late October during a closed-door hearing, spokeswoman Anna Veduta told Agence France Presse.

Russian Foreign Minister Sergei Lavrov said on Thursday that his country will support the Lebanese army's efforts to confront terrorism, considering that the presidential elections in Lebanon will unify the country.

“Russia will fortify its military cooperation with Lebanon and will beef up the defense capabilities of Lebanese Armed Forces,” Lavrov said in a joint press conference with his visiting Lebanese counterpart Jebran Bassil.