Russia has said it has no intention of invading eastern Ukraine, but a top politician from Kiev told Fox News Saturday that there is "still a huge possibility that Russia could invade and seize Ukrainian territory."
Ukrainian Prime Minister Arseniy Yatsenyuk voiced his concern as Russia's military takes positions on the country's border. Yarsenyuk, however, expressed some optimism for a diplomatic solution after an hour-long phone call Friday between President Obama and President Vladimir Putin.
"Diplomacy is always the best way forward," he said.
Yatsenyuk's concern for further Russian aggression is shared by many in the West.
U.S. Defense officials told Fox News the numbers of troops at Ukraine's border far exceeds the amount needed for a training exercise. And the fact that there is no real evidence any large-scale exercises have occurred, and that none of the troops have returned to their bases, is also concerning to U.S. observers.
Andriy Parubiy, Ukraine's top security official, estimates that Russia has 88,000 troops on the border. It is believed that an additional 50,000 troops may have flooded the region in the last few days. One Russian official dismissed the claims about a troop buildup, saying Western officials should "take a pain reliever."
The White House issued a press release the said Putin phoned Obama, who was travelling in Saudi Arabia, to discuss the situation that has put both countries on precarious footing. The invasion of Crimea is popular in Russia, but the country faces isolation from the West over its perceived aggression.
Obama urged Putin to avoid further military provocations in Ukraine, and to pull back the troops that Russia has on the Ukraine border, the statement said. Obama said Ukraine's government is pursuing de-escalation despite Russia's incursion into Crimea.
However the Russian government's account of the call has a different take, with the Kremlin asserting Putin led the conservation. It states Putin told Obama the international community needs to work together to stabilize the Ukraine situation.
The Kremlin release also says Putin drew Obama's attention to the "continued rampage of extremists" in Ukraine, saying they are intimidating citizens, law enforcement and the government.
Deep divisions between Ukraine's Russian-speaking eastern regions, where many favor close ties with Moscow, and the Ukrainian-speaking west, where most want to integrate into Europe, continue to fuel tensions.
The Crimean Peninsula, where ethnic Russians are a majority, voted this month to secede from Ukraine before Russia formally annexed it, a move that Western countries have denounced as illegitimate. Talk percolates of similar votes in other Ukrainian regions with large Russian populations, although none has been scheduled.
U.N. Secretary-General Ban Ki-moon said Friday that Putin had assured him he had no intention of making another military move into Ukraine.
That was echoed by Russia's U.N. Ambassador Vitaly Churkin, who said Putin made clear in a March 18 statement that there was not going to be any new Russian move into Ukraine.
While Putin has said Russia doesn't want a division of Ukraine, he also sought to cast it as an artificial state created by the Communists that includes historic Russian regions -- controversial statements that raise doubts about the Kremlin's intentions.
To tamp down those fears, the Russian Foreign Ministry said Friday that Moscow allowed observation flights over the border by Ukrainian, U.S., German and other Western officials. It said if any major troop concentrations had been spotted, the West wouldn't have been shy to speak about it.
Russia also kept pushing its long-held contention that ethnic minorities in Ukraine are living in fear of the new interim authorities. The Foreign Ministry said not just ethnic Russians, but ethnic Germans, Hungarians and Czechs in Ukraine also are feeling in peril.
"They are unsettled by the unstable political situation in the country and are seriously afraid for their lives," the statement said, without citing specific incidents.
Fox News' Greg Palkot and The Associated Press contributed to this report