The Ukrainian government on Monday accused Russia of fomenting unrest over the weekend in eastern Ukraine as a preamble for another military incursion, after hundreds of activists occupied the Donetsk local administration building and declared the region a sovereign republic.
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The U.S. government backed up Kiev's charges, saying there was "strong evidence" that some of the pro-Russian demonstrators who have occupied government buildings in Kharkiv, Luhansk and Donetsk were "paid and were not local residents."
White House spokesman Jay Carney said they have been encouraged by "outside forces" under pressure from Russia and called for Russia to withdraw the thousands of troops massed on Ukraine's eastern border.
Carney repeated warnings that if Russia moves into Eastern Ukraine, either "overtly or covertly," it would be a "serious escalation" that could trigger a tougher round of sanctions against Russia.
The activists asked Russian President Vladimir Putin to send in peacekeepers to protect them from the "fascist system of the Junta in Kiev," according to video footage carried by local media.
Acting Ukrainian President Oleksandr Turchynov said the wave of protests was a Russian "special operation" aimed at tearing Ukraine to pieces.
"The enemies of Ukraine are playing the Crimean scenario, but we won't let this happen," Turchynov said in a television address.
Moscow's takeover of the Ukrainian Black Sea peninsula of Crimea started with similar protests by ethnic Russians, which last month voted for accession to Russia in a referendum considered illegal by Kiev and the West.
Turchynov announced the launch of an "anti-terror operation" against activists who use weapons against authorities. He also promised a bill that envisages tougher punishments against separatists.
Separatism is "not politics but a serious crime," he explained.
In a telephone conference Monday between U.S. Secretary of State John Kerry and Russian Foreign Minister Sergei Lavrov, plans were mapped out for four-way talks in the next 10 days to include the Ukraine and the European Union, according to U.S. State Department spokeswoman Jen Psaki.
Acting Ukrainian Prime Minister Arseniy Yatsenyuk warned of a Russian invasion, telling parliament that the unrest was part of a plan "to destabilize the situation so that foreign troops cross the border and occupy territory."
A senior Russian lawmaker said that Moscow would send peacekeeping troops to Ukraine only with a UN Security Council mandate.
"To send peacekeepers to Donetsk or anywhere else upon a call from local authorities contradicts international principles," Viktor Ozerov, the chairman of the defense committee in Russia's upper house of parliament, told the Interfax news agency.
Russia refused to attend a special meeting of the Organization for Security and Cooperation in Europe, where it was supposed to explain its troop activities near the Ukrainian border, diplomats said.
Washington's OSCE envoy Daniel Baer said that the refusal to engage in dialogue "further damages Russia's stature and status in the international community."
Also on Monday, presidential candidate Yulia Tymoshenko voiced support for a federalization of Ukraine and vowed not to abolish the status of Russian as an official state language, thus meeting two of Russia's key demands.
It was the first time that Tymoshenko, a former prime minister, was in eastern Ukraine since her release from a Kharkiv prison hospital immediately after the ouster of former president Viktor Yanukovych in late February.
The Russian Foreign Ministry rejected accusations it had provoked the demonstrations and reiterated its demands for constitutional reform in Ukraine.
"Enough of the accusations against Russia, that make it responsible for all of Ukraine's ills," the ministry said in a statement.
Russian-speaking eastern Ukraine has seen mass protests since Yanukovych fled Kiev, allowing the pro-Western opposition to take over the government.
Russia says the rights of Russian speakers are being violated - an argument it used to justify the annexation of Crimea. It has not ruled out military intervention in other parts of Ukraine.
NATO said Monday that it is barring all but Russia's most senior representatives from freely accessing the alliance's headquarters in Brussels. The move follows last week's decision to freeze cooperation with Moscow over the Ukrainian crisis.
UN Secretary General Ban Ki-moon was "deeply concerned" by the escalation in eastern Ukraine over the weekend, said Farhan Haq, a spokesman for Ban. He urged those responsible to "defuse tensions" and stay calm.