The White House says Thursday's shootdown of a government helicopter by pro-Russian separatists in Ukraine indicates rebels have access "to advanced weaponry and other assistance from outside."
Spokesman Jay Carney's comments came just hours after Ukraine's acting president said rebels downed the aircraft with a shoulder-launched missile. He said 14 servicemen, including a general, died in the attack near the flashpoint eastern city of Slovyansk.
Separately, U.S. Secretary of State John Kerry raised concerns Thursday with his Russian counterpart about the ongoing violence, and about reports of foreign fighters crossing into Ukraine from Russian territory.
During a phone call, Kerry pressed Russia's Sergei Lavrov to end support for separatists in eastern Ukraine and to call on them to lay down their weapons, State Department Spokeswoman Jen Psaki told reporters during Thursday’s briefing.
In other developments, U.S. Defense Secretary Chuck Hagel noted a partial pullback of Russian troops massed along the Ukraine border - a move he called "a positive step." But he also said thousands of troops remain, and he called on Moscow to complete the full withdrawal it announced earlier this month.
Despite weeks of Kremlin denials, Russia's state-controlled news agency Ria Novosti said 33 of the estimated 50 rebel fighters killed Monday in Donetsk in a battle with Ukrainian troops came from Russia.
The news agency quoted the prime minister of the self-proclaimed Donetsk People's Republic as saying the bodies of the foreigners have been identified and returned to their relatives.
Meanwhile, a separatist leader in Slovyansk acknowledged that his group detained four European observers who have been missing since Monday.
Vyacheslav Ponomaryov said the the observers from the Organization of Security and Cooperation in Europe (OSCE) had been warned to stay away from the area, but would be freed soon.
Michael Bociurkiw, spokesman for the OSCE mission to Ukraine, told VOA the mission has not had contact with the group since Monday, but is hopeful for their release.
Bociurkiw said the team was working a few hours east of Donetsk when contact with them was lost.
A day earlier, 11 OSCE monitors were detained in the same area, but released after several hours.
He said they were monitoring human rights abuses, the erection of checkpoints and the general security situation.
According to Bociurkiw, said in the past two to three weeks, the OSCE has seen an escalation in military activity in Donetsk and the neighboring Luhansk region.
“In Luhansk right now we've documented the existence of about 40 armed groups that compose about 1,000 individuals and our monitors estimate that they could recruit another 4,000 more,” said Bociurkiw.
Swiss OSCE chairman Didier Burkhalter said in a statement Wednesday the detentions are undermining the group's work and amounted to "acts of sabotage" against international efforts to help Ukraine overcome the months-long crisis.
Russia turns up rhetoric
Expressing “deep concern” and warning of a “catastrophe” in Ukraine, Moscow called on Kyiv Thursday to stop its military operation in the eastern part of the country.
“Once again, we demand from the Kyiv authorities to stop the fratricidal war, start a real national dialogue with all political forces and representatives of the country’s regions to find a way out of the deepening crisis in Ukraine,” Russia’s Foreign Ministry said in a statement.
Calling for the implementation of the April 17 Geneva Accords on de-escalating the crisis in Ukraine, Moscow also urged Western countries to use all of their influence “to stop Ukraine’s slide toward a national catastrophe.”
The Ukrainian government and President-elect Petro Poroshenko have defied Moscow’s calls accusing it of fueling the very separatist unrest that has caused the current crisis. Russia denies the charge.
Meanwhile, a top advisor for President Vladimir Putin accused the U.S. of trying to stoke a military conflict between Europe and Russia over Ukraine for its own economic benefit.
“Now they [the United States] are unfurling a war in Ukraine, after organizing a coup and putting their own people in charge, to use Ukraine as a detonator against Russia and Europe,” said Sergei Glazyev, an economic adviser to Putin. The U.S. has dismissed similar charges as groundless.
Glazyev said that the U.S. has benefited from past wars in Europe.
Official election results
Petro Poroshenko officially won Ukraine’s presidential elections in the first round, garnering 54.7 percent of all votes cast, the country’s Central Election Commission announced.
It posted election results on its website Thursday saying it had completed the vote count.
Former prime minister Yulia Tymoshenko came in second with 12.81 percent of the vote. Twenty other candidates had competed in Sunday’s poll.
Of 18 million voters who took part in the election nearly ten million supported Poroshenko, a confectionery tycoon and former foreign minister. Turnout was listed at close to 60 percent.
According to official results, Poroshenko, 48, garnered a plurality of votes in all of Ukraine’s regions, even in restive Donetsk and Luhansk.
His inauguration will take place on the “Maidan,” Kyiv’s Independence Square, a statement by Ukraine’s acting president said. It was the site of massive anti-government protests, which toppled Moscow-backed president Viktor Yanukovych in February.
No date has been officially announced.
Poroshenko is due to participate in D-Day observances in France next week, which will also be attended by other world leaders, including Presidents Barack Obama and Vladimir Putin.
With a threat of Moscow cutting off gas supplies to Ukraine looming, talks between Russia, Ukraine and the European Commission, will go ahead on Friday, the EU executive body announced Thursday.
The Commission has already brokered two rounds of talks following Moscow's threat to stop supplying Ukraine with gas if it fails to make a pre-payment for June supplies by June 2.
Ukraine wants the price for 2014 to be agreed before it starts making any payments, including on its debt, which Russia’s Gazprom says will have risen to $5.2 billion by June 7.
Kyiv has countered that Gazprom owes it around $1 billion for gas following Russia's annexation of Crimea.
Ukraine is also the transit country for about half of the gas Russia sells to the European Union, so the dispute threatens supplies to Europe.
VOA's Gabe Joselow contributed to this story from Kyiv. Some reporting by Reuters, AP.