Ukraine has relaunched military operations against pro-Kremlin separatists, hours after US Vice President Joe Biden ended a two-day visit to Kiev in which he warned Russia over its actions in the former Soviet republic.
The US Defence Department at the same time announced it was sending 600 troops to neighbouring Poland and to Baltic countries for 'exercises'.
Russia already has tens of thousands of its troops massed on Ukraine's eastern border.
The latest moves underscored the severity of the crisis that has brought East-West relations to their most perilous point since the end of the Cold War.
Ukraine's acting president, Oleksandr Turchynov, said he was ordering the military to restart operations against the rebels after the discovery of two 'brutally tortured' bodies in the eastern rebel-held town of Slavyansk.
One of them, he said, was that of a recently kidnapped local councillor from a nearby town who belonged to his party.
In a further slide back towards violence, which many fear could tip into civil war, a Ukrainian reconnaissance plane was hit by gunfire while flying above Slavyansk.
The Antonov An-30 propellor-driven plane received several bullet impacts, but safely made an emergency landing and none of its crew members were hurt, said the defence ministry in Kiev.
The Organisation for Security and Co-operation in Europe, which has monitors in the country, also said that rebels had abducted a police chief in the town of Kramatorsk - calling it the sort of 'provocative' action that 'can only worsen the existing tensions and contribute to further violence'.
Pro-Moscow militants had taken over Kramatorsk's police station late on Monday, extending their grip from the already occupied town hall.
Kiev, Washington and many EU countries see Moscow as pulling the strings in the Ukrainian separatist insurgency.
Biden, in his news conference after meeting the Kiev authorities, warned Russia of isolation if it continues to try to 'pull Ukraine apart', underlining a US threat to impose more sanctions on Moscow.
'We have been clear that more provocative behaviour by Russia will lead to more costs and to greater isolation,' said the vice president.
But Russia says Kiev's new leaders - whom it regards as illegitimate - are to blame for the collapse of the accord.
In Moscow, Russian Prime Minister Dmitry Medvedev dismissed the US threat of new sanctions.
'I am sure we will be able to minimise their consequences,' he said in a televised speech to the Russian parliament.
However he acknowledged that Russia's economy was facing an 'unprecedented challenge'.
Russia's finance ministry said Monday the energy-rich nation could tip into 'technical recession' over the next three months. Last week it warned Russia was facing the toughest economic conditions since 2009, when a serious slowdown occurred.