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Kiev said the overnight seizure of public buildings in three cities in eastern Ukraine's mainly Russian-speaking industrial heartland were a replay of events in Crimea, the Black Sea peninsula Moscow seized and annexed last month.
"An anti-Ukrainian plan is being put into operation ... under which foreign troops will cross the border and seize the territory of the country," Prime Minister Arseny Yatseniuk said in public remarks to his cabinet. "We will not allow this."
Pro-Russian protesters seized official buildings in the eastern cities of Kharkiv, Luhansk and Donetsk on Sunday night (local time), demanding that referendums be held on whether to join Russia like the one that preceded Moscow's takeover of Crimea.
Acting President Oleksander Turchinov, in a televised address to the nation, said Moscow was attempting to repeat "the Crimea scenario". He added that "anti-terrorist measures" would be deployed against those who had taken up arms.
US Secretary of State John Kerry told Russian Foreign Minister Sergei Lavrov in a phone call that Washington was watching events in eastern Ukraine with great concern and any further moves by Moscow to destabilize Ukraine would "incur further costs for Russia."
Kerry "called on Russia to publicly disavowed the activities of separatists, saboteurs and provocateurs" in Ukraine, the State Department said. The two discussed convening direct talks in the next 10 days between Ukraine, Russia, the United States and the European Union to defuse tensions.
WHITE HOUSE WARNING
The White House warned Russian President Vladimir Putin against moving "overtly or covertly" into eastern Ukraine and said there was strong evidence that pro-Russian demonstrators in the region were being paid.
Lavrov told Kerry that constitutional reform was required to resolve the crisis. Russia says this would give Ukraine's regions more powers, as it believes the rights of ethnic Russians in Ukraine are being violated.
In an article on the website of Britain's Guardian newspaper, Lavrov denied Russia was destabilising Ukraine and accused the West of the "groundless whipping-up of tension".
Separately, he warned authorities in Kiev against any use of force against pro-Russian demonstrators.
Police said they cleared the protesters from the building in Kharkiv, but in Luhansk the demonstrators had seized weapons.
In Donetsk, home base of deposed Moscow-backed President Viktor Yanukovich, about 120 pro-Russia activists calling themselves the "Republican People's Soviet of Donetsk" seized the chamber of the regional assembly.
An unidentified bearded man read out "the act of the proclamation of an independent state, Donetsk People's Republic" in front of a white, blue and red Russian flag.
"In the event of aggressive action from the illegitimate Kiev authorities, we will appeal to the Russian Federation to bring in a peacekeeping contingent," ran the proclamation.
The activists later read out the text by loud hailer to a cheering crowd of about 1,000 outside the building.
Ukrainian Interior Minister Arsen Avakov said on Monday the main regional administration building in Kharkiv had been cleared of "separatists". But police in Luhansk said protesters occupying the state security building there had seized weapons. Highway police closed off roads into the city.
"Unidentified people who are in the building have broken into the building's arsenal and have seized weapons," police said in a statement. Nine people had been hurt in the disturbances in Luhansk.
Putin announced on March 1, a week after Yanukovich was overthrown, that Moscow had the right to take military action in Ukraine to protect Russian speakers, creating the biggest confrontation between Moscow and the West since the Cold War.
The United States and EU imposed mild financial sanctions on some Russian officials over the seizure of Crimea and have threatened much tougher measures if Russian troops, now massed on the frontier, enter other parts of Ukraine.
Western European governments have hesitated to alienate Russia further, fearing for supplies of Russian natural gas, much of which reaches EU buyers via pipelines across Ukraine. Ukraine's own dependence on Russian gas gives Moscow strong leverage, especially over Ukraine's eastern industrial areas.
Russia's gas monopoly Gazprom said it had received no payments from Ukraine for money owed for gas. It has given Kiev until midnight (2000 GMT) to reduce a $2.2 billion gas debt, although it has not said what it will do if Kiev misses the deadline. In previous years, gas disputes between Moscow and Kiev have hurt supplies to Europe.
In Vienna, Russia did not attend a meeting on Ukraine of the Organisation for Security and Cooperation in Europe. The U.S. envoy to the OSCE, Daniel Baer, said Moscow needed to explain why tens of thousands of its troops were massed on the border.
NATO has halted cooperation with Russia. The Western military alliance announced on Monday it would now restrict access to its headquarters by Russian diplomats apart from Moscow's ambassador, his deputy and two support staff.
Mainly Russian-speaking eastern Ukraine, densely populated and producing much of the country's industrial output, has seen a sharp rise in tension since Yanukovich fled the country, and Kiev has long said it believes Moscow is behind the unrest.
Pro-Russian protesters briefly held public buildings in the east early last month and three people were killed in clashes in mid-March. But trouble had subsided until Sunday.
Unlike in Crimea, where ethnic Russians form a majority, most people in the east and south are ethnically Ukrainian, although they speak Russian as a first language. Eastern oligarchs who once backed Yanukovich have thrown their weight behind the government in Kiev, and the unrest there is a test of their ability to assert control.
Yanukovich, in exile in Russia, has called for referendums across Ukrainian regions on their status within the country.
Yatseniuk said that though much of the unrest had died down in eastern Ukraine in the past month there remained about 1,500 "radicals" in each region who spoke with "clear Russian accents" and whose activity was being coordinated abroad. But he said Ukrainian authorities had drawn up a plan to handle the crisis.
Avakov accused Putin on Sunday of orchestrating the "separatist disorder" and promised that disturbances would be brought under control without violence.
Russia has been pushing internationally a plan proposing the "federalisation" of Ukraine in which regions of the country of 46 million would have broad powers of autonomy.
Ukraine, drawing up its own plan for "de-centralisation" in which municipalities would retain a portion of state taxes, says the Russian proposal is aimed at carving it up.
Ukraine's defence ministry said a Russian marine had shot and killed a Ukrainian naval officer in Crimea on Sunday night.