WASHINGTON (AP) -- Secretary of State John Kerry agreed Monday to meet with top diplomats from Russia, Ukraine and the European Union in a new push to calm tensions in eastern Ukraine, as the White House threatened further sanctions if Moscow intervenes.
Kerry's meeting with his counterparts will take place in the next 10 days, the State Department said, and comes amid mounting U.S. concern that Russia could build on its recent annexation of Crimea by trying to split off other areas of Ukraine. To that end, the White House issued a warning to Russian President Vladimir Putin.
"If Russia moves into Eastern Ukraine, either overtly or covertly, this would be a very serious escalation," White House spokesman Jay Carney said, adding that the U.S. is "prepared to impose further sanctions on sectors of the Russian economy should the situation escalate."
Pro-Russian activists who have barricaded inside a government building in eastern Ukraine proclaimed the region independent, while calling for a referendum on seceding from Ukraine -- a chain of events with ominous echoes of the prelude to Russia's annexation last month of Crimea. The Ukrainian government has vowed to quell the unrest despite the tens of thousands of Russian troops stationed along their joint border.
Making clear the U.S. considers Russia responsible for inciting unrest in eastern Ukraine, Carney said there's strong evidence that some pro-Russian protesters who have taken over government buildings in eastern Ukraine were paid, and are not local residents.
"What's clear is that this is a result of increased Russian pressure on Ukraine. And we see it in the troops that have massed on the border," Carney said, urging Putin to remove those troops from the border.
To those advocating a tougher U.S. response, the White House's admonitions to Moscow were again insufficient. Sen. John McCain, R-Ariz., said the unrest in eastern Ukraine appeared to come "straight out of President Putin's playbook."
"As the risk of further Russian aggression against Ukraine appears to be growing, it is outrageous that the Obama administration still has not met the Ukrainian government's request for modest military assistance," McCain said.
The White House has resisted calls to send advanced weaponry to Ukraine, arguing that the goal is to de-escalate the situation rather than inflame it.
In a phone call earlier Monday, Kerry told his Russian counterpart, Foreign Minister Sergey Lavrov, that the U.S. was watching events in an array of eastern Ukrainian cities with "great concern" and skepticism that they were spontaneous, the State Department said. Kerry pointed out that Russian intelligence operatives working in Ukraine had recently been arrested and called on Russia's government to disavow the actions of separatists.
Lavrov, for his part, emphasized a "persistent need for a deep and transparent constitutional reform with full involvement of all political forces and regions, particularly amid the escalating protests in southeastern Ukraine," Russia's foreign ministry said in a statement.
He also underlined the "importance of a real dialogue on the federative structure of Ukraine and sealing its bloc-free status" -- a nod to the Kremlin's preference that Ukraine abstain from joining NATO.
Associated Press writer Vladimir Isachenkov in Moscow contributed to this report.
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