The crisis in Ukraine deepened further overnight following the departure of Vice President Joe Biden from Kiev after a two-day visit.
In a late-night phone call, Secretary of State John Kerry told the 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 peace accord brokered in Geneva, which many hoped would avert Ukraine's slide into civil war.
Moscow maintains the accord was ruptured by ultranationalists -- who were involved in months of protests that led to the ousting in February of Ukraine's pro-Kremlin president, Viktor Yanukovych -- who killed rebels in an attack on Sunday near the eastern town of Slavyansk.
On Wednesday morning, Russia's defense ministry announced on state news wire Interfax that the navy had launched snap military exercises involving its fleet in the Caspian Sea.The drill will last seven days and involve around 10 naval vessels and 400 sailors. The Caspian Sea is bordered by Iran, Russia, Kazakhstan, Turkmenistan and Azerbaijan -- a region that is crisscrossed by oil and natural gas pipelines.
Ukraine relaunched military operations against pro-Kremlin separatists late on Tuesday, hours after Biden ended his visit to Kiev in which he warned Russia over its actions in the former Soviet republic.
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 Slavyansk.
One of them, he said, was that of a recently kidnapped local councilor from a nearby town who belonged to his party.
The U.S. Defense Department at the same time announced it was sending 600 troops to neighboring Poland and to Baltic countries for "exercises."
Russia already has tens of thousands of its troops massed on Ukraine's eastern border and has boosted the number of snap military drills since tensions have risen with neighbouring Ukraine.
NATO has said Russia's military exercises in its western military district have boosted its troop presence to around 40,000 near the border with Ukraine.
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.
In a further slide back toward 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 was hurt, said the defence ministry in Kiev.
The Organization 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".
Biden, in his news conference after meeting the Kiev authorities, warned Russia of isolation if it continued trying to "pull Ukraine apart," underlining a U.S. threat to impose further sanctions on Moscow.
"We have been clear that more provocative behavior by Russia will lead to more costs and to greater isolation," said the vice president.
In a separate development, Sweden, which is not a NATO member, announced on Tuesday it was increasing defense spending because of the "deeply unsettling development in and around Ukraine". It plans to boost its fleets of fighter jets and submarines.