President Barack Obama called on Russia on Friday to pull back tens of thousands of troops massed along its border with Ukraine, as U.S. officials said they have grown increasingly concerned that the size and makeup of the force could portend a new Russian attempt to annex swaths of its neighbor.

President Barack Obama called on Russia on Friday to pull back tens of thousands of troops massed along its border with Ukraine, as U.S. officials said they have grown increasingly concerned that the size and makeup of the force could portend a new Russian attempt to annex swaths of its neighbor.

In recent days, Russia has deployed sustainment units in addition to combat forces to border areas and has taken steps to conceal some of the contingents, U.S. officials said. Speaking on the condition of anonymity to discuss intelligence assessments, U.S. officials added that they have seen no evidence to corroborate Russia’s assurances that the soldiers are there solely to conduct training exercises.

In a phone call with Russian President Vladimir Putin on Friday, Obama urged Russia to "avoid further provocations, including the buildup of forces on its border with Ukraine."

Putin called Obama on Friday to discuss how to pursue a "diplomatic resolution" to the crisis in Ukraine, the White House announced. Obama suggested that Russia "put a concrete response in writing" to a U.S. proposal presented to the Russians this week, and the presidents agreed that Secretary of State John Kerry and Russian Foreign Minister Sergei Lavrov "would meet to discuss next steps," the White House said.

A Kremlin account of the call focused less on the prospects for diplomacy and more about stabilizing a situation Moscow blamed on "extremists."

Putin "drew Barack Obama’s attention to continued rampage of extremists who are committing acts of intimidation towards peaceful residents, government authorities and law enforcement agencies in various regions and in Kiev with impunity," according to the Kremlin summary. The Russian president said Russia was interested in "examining possible steps the global community can take to help stabilize the situation."

The United States and other Western governments have rejected repeated charges by Moscow that ethnic Russians in eastern and southern Ukraine are under threat, and U.S. and Ukrainian officials have said violent incidents are often fomented by provocateurs directed by Russia’s secret services.

In Washington, Sens. John McCain, R-Ariz., and Lindsey Graham, R-S.C., called on the Obama administration to heed Ukraine’s requests for military equipment, a step the White House has so far declined to take because officials fear it could heighten the chances of armed conflict between Russia and Ukraine.

"The West should act now to help them," the senators said in a statement. "Giving victims of aggression some better means to defend their sovereign territory against further aggression is simply the right and decent thing to do."

The lawmakers said Russia’s annexation of Crimea this month should serve as a "wake-up call" to the North Atlantic Treaty Organization, and they argued that the alliance needs to "urgently conduct contingency planning and military exercises" to deter further aggression. They said Moldova and Georgia, two other former Soviet republics, are vulnerable.

Putin, in his phone call with Obama, raised concerns about the Transnistria region of Moldova, bordering Ukraine, where separatists have declared their independence and a desire to join Russia. "Transnistria is essentially experiencing a blockade, which significantly complicate the living conditions for the region’s residents," Putin said, according to the Kremlin account.

A senior NATO official earlier this week warned that Russia might order its troops to cross Ukraine and enter Moldova.

Russian officials reiterated Friday that the troops along the border are there solely for training exercises. Foreign Ministry spokesman Alexander Lukashevich said that international observers had visited the area and did not report seeing evidence of a looming invasion.

Officials at the Pentagon and intelligence agencies have closely studied Russian troop movements this week, seeking insight into Moscow’s strategy.

"We remain concerned about their intent," Rear Adm. John Kirby, a Pentagon spokesman, said Friday. "We don’t have perfect visibility on that intent. We’ve been assured they’re there simply to conduct training exercises. Our expectation is that they are going to live up to their word."

U.S. intelligence agencies think the troop deployment to the border currently includes tens of thousands of soldiers, an American official said. But government analysts do not yet have a strong indication of Putin’s plans, the official said.

"We are seeing activity and keeping policymakers informed," said Shawn Turner, spokesman for the director of national intelligence.

American and European leaders were taken aback by the speed with which Russia annexed Crimea, a province where many ethnic Russians welcomed Moscow’s embrace. Seeking to prevent Russia from reaching deeper into Ukraine, the Obama administration and its allies have imposed sanctions on senior Russian officials and threatened to take steps that would harm Russia’s key sectors, including energy.

Alexander Malevany, deputy head of Russia’s security service, the FSB, said at a Kremlin ceremony Friday that Moscow is taking steps to counteract what he described as malign meddling by the West.

"The lawful desire of the peoples of Crimea and eastern Ukrainian regions is causing hysteria in the United States and its allies," he said. "They are taking measures designed to have a destructive impact on the sociopolitical and economic situation in our country. They have clearly demonstrated their desire to weaken Russia’s influence on a vitally important area."

In an interview with CBS News on Friday, Obama said Moscow is misreading U.S. foreign policy.

"We have no interest in encircling Russia, and we have no interest in Ukraine beyond letting the Ukrainian people make their own decisions about their own lives," he said.

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Scott Wilson in Riyadh, Saudi Arabia, Will Englund in Moscow and Adam Goldman in Washington contributed to this report.