The presidents of Russia, Germany, France and Ukraine held a joint phone call on Wednesday to discuss the situation in Ukraine, Kremlin spokesman Dmitry Peskov told reporters.
Russian President Vladimir Putin on Tuesday asked lawmakers to revoke a resolution allowing him to invade Ukraine in a shock turnabout that Kiev hailed as his "first practical step" towards helping defuse the crisis.
Fighting flared between Ukrainian and pro-Moscow separatist forces, both sides reported on Sunday, further straining a unilateral ceasefire declared by Ukraine as Russian President Vladimir Putin pressed Kiev to talk to the rebels.
Pro-Russian separatists attacked Ukrainian posts on the border with Russia and a military base and tried to storm an airforce base overnight into Saturday, government forces said, putting a Ukrainian unilateral ceasefire under pressure.
President Barack Obama on Tuesday unveiled a $1 billion US security plan for eastern Europe aimed at allaying fears over a resurgent Kremlin and the escalating pro-Russian uprising in ex-Soviet Ukraine.
Pro-Russia insurgents in Ukraine's Donetsk region declared independence on Monday and asked to join Russia - a day after holding a hastily arranged vote on separatism that Ukraine's government and its western allies said violated international law.
President Vladimir Putin flew in to Crimea on Friday, marking the Soviet victory in World War Two and proclaiming the success of the peninsula’s seizure from a Ukraine that Russia says has been taken over by fascists.
More than 3,000 pro-Russian protesters stormed the regional administration building in the eastern Ukraine city of Lugansk on Tuesday, AFP journalists at the scene reported.
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.
US secretary of state John Kerry told Russian foreign minister Sergei Lavrov in a telephone call on Tuesday that Washington would impose more sanctions on Russia if tensions did not de-escalate in eastern Ukraine, a senior US official said.
Russia said its military is massed on Ukraine's doorstep and warned against further US sanctions as a deal struck with Washington appeared to stall because of intransigence by Moscow-backed rebels in the former Soviet republic.