Russian President Vladimir Putin on Monday called on US President Barack Obama to use Washington's influence in Kyiv "to prevent the use of force and bloodshed in Ukraine."

On Sunday, Ukraine's pro-Western government had issued an ultimatum, calling on armed pro-Russian separatists to vacate municipal buildings they've occupied in the country's east or face military force. So far, Kyiv has not followed through with its threat.

During his Monday phone converation with the US president, Putin dismissed as "unfounded" the allegations that Russian agents had instigated the latest unrest in eastern Ukraine, according to the Kremlin.

But Obama told Putin that Moscow must press the pro-Russian gunmen in eastern Ukraine to lay down their arms. The US president warned that Russia would face further isolation if it did not end its support for the separatists.

"The president noted Russia's growing political and economic isolation as a result of its actions in Ukraine and made clear that the costs Russia has already incurred will increase if those actions persist," the White House said in a news release.

Earlier in the day, the White House accused Moscow of orchestrating the armed, pro-Russian uprisings that have gripped several cities in eastern Ukraine over the past week. The EU and the Ukrainian government have also implicated the Kremlin in the unrest.

"The evidence is compelling that Russia is supporting these efforts and involved in these efforts," said White House spokesman Jay Carney. "You saw this coordinated effort in a number of cities across eastern Ukraine all at once that sure didn't look organic to observers from the outside."

US confirms CIA director visit

Carney also confirmed that the director of the Central Intelligence Agency (CIA), John Brennan, visited Kyiv over the weekend. The Russian news agency Interfax, quoting an unnamed official, reported that Brennan had encouraged the Ukrainian government to use force against separatists in the country's east.

Russian Foreign Minister Sergei Lavrov demanded an explanation from Washington about why the CIA director had visited Kyiv. In an unusual move, Carney said that the White House had decided to reveal Brennan's visit to dispel "false clams" by Moscow.

"Senior-level visits of intelligence officials are a standard means of fostering mutually beneficial security cooperation, including US-Russian intelligence collaboration going back to the beginnings of the post-Cold War era," Carney said.

"To imply that US officials meeting with their counterparts is anything other than in the same spirit is absurd," he continued.

Russian jet buzzes US destroyer

Meanwhile, the Pentagon said that a Russian Su-24 fighter jet had buzzed the USS Donald Cook, an American cruise missile destroyer stationed in the Black Sea off the coast of NATO member state Romania.

"This aircraft did not respond to multiple queries and warnings from Donald Cook," said Colonel Steven Warren, a spokesman for the Pentagon.

"This provocative and unprofessional Russian action is inconsistent with international protocols and previous agreements on a professional interaction between our militaries," Warren continued.

Tensions have escalated in Ukraine over the past week after pro-Russian gunmen seized control of government buildings in nine cities in the country's east. In the city of Slovyansk, a Ukrainian security officer was shot dead on Sunday during an exchange of gunfire with pro-Russian gunmen.

slk/jm (AP, AFP, dpa, Reuters)