Ukraine's interim prime minister said on Thursday his country was entering its "most dangerous 10 days" since independence in 1991 and was struggling to counter pro-Russian separatists on the verge of taking over the industrialized eastern heartland.
Arseniy Yatseniuk, in an interview with the Financial Times, accused Moscow of plotting to foment more clashes during the May Day holidays when nostalgia for Soviet victories and achievements tends to peak.
Pro-Russians strengthened their grip on the east of Ukraine on Thursday, storming the regional prosecutor's office in the town of Donetsk driving the police out and ransacking the building. The Kiev authorities fear the secessionists will put on a bigger show of strength on May 9, the commemoration of the Soviet Union's victory over Nazi Germany.
On May 11, pro-Russian separatists who have seized government buildings in about a dozen cities in the Donetsk and Lugansk regions, plan to hold a referendum on independence and later unification with Russia.
"They will play on this Soviet-style legacy and try to provoke and artificially make clashes," Mr Yatseniuk said. "They … usually have 10 to 20 well-trained Russian agents who storm the buildings, then grab indigenous protesters, disappear and move to another city."
German chancellor Angela Merkel telephoned Vladimir Putin, the Russian president, in the latest western attempt to dissuade Moscow from stoking the crisis. Ms Merkel will hold talks on the Ukrainian crisis with President Barack Obama in Washington on Friday.
Mr Putin said Ukraine's government should defuse the political crisis by withdrawing its troops from southeastern regions and launching a broad constitutional reform process. He was quoted by the Kremlin as telling Ms Merkel that those steps were "the most important thing now" to resolve the crisis.
Ukraine's interim authorities introduced conscription on Thursday in the wake of launching an operation against the rebels last week, which has made little apparent progress.
Speaking in Kiev, Mr Yatseniuk said: "It's clear to me, President Putin has no limits. Putin's idea is to restore the might of the former Soviet Union and become emperor of a new-style Soviet empire." The West, he added, "got this message … too late though … after the Crimea invasion."
He described the risk of broader Russian military intervention as high and described Ukraine's central government as falling into "a trap".
"On the one hand, the majority of Ukrainians are pressing on the acting president and security service to conduct the anti-terrorist operation and to bring these terrorists to justice. On the other hand, if you start this kind of very tough operation, you will definitely have civilian casualties. And this is the perfect excuse for President Putin to say look, these ultranationalists kill Russian-speaking people. We need to protect them," he said.
"The intel we got is very clear," he said. "Russia is entirely ready to cross the border as they did in Crimea."
Mr Yatseniuk insisted Kiev had not yet lost all authority over the Donetsk and Lugansk regions, but said pro-Russians had taken control of "up to 10" cities. He said the central government still intended to hold a presidential election on May 25, and held out the prospect of a referendum on greater regional autonomy and Russian language rights.
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Mr Yatseniuk outlined the daunting challenges facing him and the interim administration: "a massive Russian troop presence on the border, well-trained Russian spies in Donetsk, Lugansk and even southern Ukraine, along with huge economic turmoil and a still very corrupt system [of government]. You can't eradicate [this] in two weeks."
Adding to its potential problems, the International Monetary Fund said it would be forced to rethink the scale of its $17bn bailout for Ukraine were Kiev to lose control of the east.