Treasury Secretary Jack Lew and Ukrainian Finance Minister Oleksandr Shlapak signed a $1 billion loan guarantee in a ceremony at the U.S. Treasury Department on Monday. (Reuters)

President Obama and Russian leader Vladi­mir Putin spoke by phone on Monday in an attempt to reduce rising ethnic tensions in eastern Ukraine and avert a feared Russian military incursion, as U.S. officials warned of further sanctions if the situation worsens.

A White House statement said the call was made at “Moscow’s request,” and that Obama “expressed grave concern about Russian government support for the actions of armed, pro-Russian separatists who threaten to undermine and destabilize the government of Ukraine.”

“The president emphasized that all irregular forces in the country need to lay down their arms, and he urged President Putin to use his influence with these armed, pro-Russian groups to convince them to depart the buildings they have seized,” the statement said. “The president reiterated the importance of Russia withdrawing its troops from Ukraine’s border in order to defuse tensions.”

In the Kremlin version of the call, the leaders’ first since late March, Putin urged Obama to bring as much American pressure to bear as possible to head off the use of force and bloodshed in Ukraine. The account underscored the starkly differing ways that U.S. and Russian officials characterize the crisis in Ukraine, a gulf that so far is undermining a search for a solution.

Noting that Obama expressed concern about Russian intervention in southeast Ukraine, the Kremlin statement said Putin said “such speculation is based on false information.”

Putin, the statement said, attributed the unrest to “the unwillingness and inability of the leadership in Kiev to take into account the interests of the [ethnic] Russian and Russian-speaking population.”

The call came amid heightened volatility in Ukraine, as armed groups that U.S. officials say are sponsored by Moscow occupy police stations and government buildings in the Donetsk region near the Russian border. The leaders have talked several times during the crisis, but so far the personal diplomacy has failed to ease conditions on the ground.

In recent days, the Ukrainian government has deployed forces to the region, setting a deadline for the militants to abandon the buildings or face attack. That deadline passed Monday without an operation, and the Obama administration continued to commend the Ukrainian government for its “restraint” during the weeks-long crisis.

Obama has been calling European leaders to coordinate any next steps against Russia should the crisis escalate.

On Monday, he spoke with French President François Hollande, and a White House account of the call said they discussed “the worsening situation in eastern Ukraine, where pro-Russian separatists, seemingly with support and coordination from Moscow, have intensified their campaign to undermine and destabilize the Ukrainian government.”

Obama has already authorized sanctions that would target important sectors of Russia’s economy. But administration officials have said those will only be imposed, ideally in tandem with European nations, if Russia, which has thousands of troops deployed along the Ukrainian border, intervenes further.

“We are actively evaluating what is happening in Ukraine, what actions Russia has taken, what transgressions they engaged in,” White House press secretary Jay Carney told reporters Monday. “And we are working with our partners and assessing for ourselves what response we may choose.”

Carney said there was “overwhelming evidence” of Russian involvement in the unrest in eastern Ukraine. Asked why those Russian actions alone would not trigger new sanctions, Carney said, “there will be further costs imposed on Russia.”

“And certainly, if they go further down the road of attempting to destabilize Ukraine, rather than choosing the path of de-escalation, the cost will continue to grow,” he said.

Administration officials also confirmed Russian media reports that CIA Director John O. Brennan visited Kiev over the weekend, saying it was part of his scheduled trip to Europe.

U.S. officials typically do not comment on the CIA director’s travel. But State Department spokeswoman Jennifer Psaki said the administration decided to do so in this case to dispute information circulated by the Russian government.

“Senior-level visits of intelligence officials are a standard means of fostering a mutually beneficial security cooperation,” Psaki said, adding that reports that Brennan encouraged the Ukrainian government to “conduct tactical operations” against the pro-Russian demonstrators are “completely false.”

Officials at the Pentagon on Monday protested what they described as a provocative flyover by a Russian attack aircraft that hovered near a U.S. Navy ship for 90 minutes on Saturday.

A military official who was not authorized to speak on the record said the Russian aircraft, a Sukhoi Su-24, made “multiple close-range, low-altitude passes” near the USS Donald Cook, a guided missile destroyer, while the ship was conducting “routine operations” in international waters.

The ship had just entered the Black Sea, and its arrival there was celebrated in Kiev as a show of American support for Ukraine.

The crew aboard the American ship issued several queries to the aircraft using communication systems that are accessible to Russian military personnel. They went unanswered. The military official said the Russian plane appeared to have been unarmed.

“This was a provocative action,” the official said. “It is inconsistent with international protocols and what you would expect from a professional military.”

The top military leader at NATO, U.S. Air Force Gen. Philip Breedlove, is expected to give the alliance and the Obama administration a fresh set of options this week of steps the coalition could take to support Ukraine and other countries in the region.

Will Englund in Moscow contributed to this report.