President Barack Obama on Friday said Russia's troop buildup on the Ukraine border was out of the ordinary and called on Moscow to pull its military back and begin talks to defuse tensions, Reuters reported.
"You've seen a range of troops massing along that border under the guise of military exercises," he told CBS "This Morning" in an interview in Vatican City. "But these are not what Russia would normally be doing."
Obama said the moves might be no more than an effort to intimidate Ukraine, but also could be a precursor to other actions.
"It may be that they've got additional plans," he said.
The vast majority of residents of Crimea - 96 percent - voted to secede from Ukraine and join Russia, in a referendum held March 16.
Most countries refused to recognize the referendum's results.
On March 18 Russia and Crimea signed an agreement on Crimea as well as Sevastopol city joining the Russian Federation.
A change of power took place in Ukraine on Feb.22.
The Verkhovna Rada (parliament) of Ukraine ousted President Viktor Yanukovych from the power, changed the constitution and scheduled presidential elections for May 25.
Yanukovych said he was forced to leave Ukraine under the threat of violence, and he remains the legally elected head of state.
A number of provinces in eastern and southern Ukraine, as well as the Crimea did not recognize the legitimacy of the Rada and decided on possibility of holding a referendum on the future fate of the regions.