Ukraine relaunched military operations against pro-Kremlin separatists late Tuesday, 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, late Tuesday 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 Cooperation 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 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.
And, in a phone call late on Tuesday, US Secretary of State John Kerry told Russian Foreign Minister Sergei Lavrov of his "deep concern over the lack of positive Russian steps to de-escalate" the crisis in eastern Ukraine, a State Department official said.
Kerry also called on Russia to "tone down escalatory rhetoric".
But Russia says Kiev's new leaders -- whom it regards as illegitimate -- are to blame for the collapse of the accord.
It says ultra-nationalists who were involved in months of protests that ousted Ukraine's pro-Kremlin president Viktor Yanukovych in February killed rebels in an attack Sunday near the eastern town of Slavyansk.
A funeral for the militants was held on Tuesday. Bells rung loudly from Slavyansk's Orthodox church and women wept as three coffins were carried out.
Biden urges Russian pullback
Biden called on Russia to pull back its forces from the border, and to reverse its annexation last month of Ukraine's Crimea peninsula.
"We in the United States stand with you and the Ukrainian people," Biden said in a joint news conference with Ukrainian Prime Minister Arseniy Yatsenyuk.
He added that the United States was stepping up to help Ukraine lessen its dependence on Russian gas, fight corruption, and prepare for a May 25 election to choose a new president.
Yatsenyuk responded that Kiev valued the US support against what he said was a Russia "acting like an armed bandit".
The Pentagon, announcing the dispatch of 150 troops to Poland and 450 to Estonia, Lithuania and Latvia in coming days, said it was sending a "message to Moscow".
Rear Admiral John Kirby told reporters in Washington that "since Russia's aggression in the Ukraine, we have been constantly looking at ways to reassure our allies and partners".
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.
Divide over further sanctions
The European Union, meanwhile, is divided on going further with its own sanctions on Moscow, with some member states worried that increased punishment could jeopardise supplies of Russian gas.
As the crisis deepens, the insurgents in Ukraine's east remain firmly entrenched in public buildings they have occupied for more than a week.
In the town of Lugansk, close to the Russian border, protesters pledged to hold their own local referendum on autonomy on May 11, the Interfax-Ukraine news agency reported.
Although highly trained military personnel, whose camouflage uniforms are stripped of all insignia, are helping the rebels secure the some 10 towns they hold, Russian President Vladimir Putin denies they are Russian special forces.
But the US State Department released images Monday it claims proves some of the armed "separatists" in Ukraine are actually Russian military or intelligence officers.
In a separate development, Sweden, which is not a NATO member, announced Tuesday it was increasing defence spending because of the "deeply unsettling development in and around Ukraine". It plans to boost its fleets of fighter jets and submarines.