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Russia called for the adoption of a national unity deal in Ukraine even as it tightened its stranglehold over Crimea, an audacious combination of diplomacy and escalating military pressure. The U.S. and European Union floundered for solutions — whileglobal markets panicked over the prospect of violent upheaval in the heart of Europe.

Then came dramatic claims from Ukraine that Russian troops had issued an ultimatum for two Ukrainian warships to surrender or be seized — prompting Ukraine’s acting president to accuse Russia of “piracy.”

Four Russian navy ships in Sevastopol’s harbour were blocking Ukraine’s corvette Ternopil and the command ship Slavutych, Ukrainian authorities said. Acting president Oleksandr Turchynov said commanders and crew were “ready to defend their ships … They are defending Ukraine.”

Vladimir Anikin, a Russian defence ministry spokesman in Moscow, dismissed the report of a Russian ultimatum as nonsense but refused to elaborate.

The Obama administration says any Russian threat to Ukraine’s navy would be a “dangerous escalation” of an extremely tense situation.

Fears grew that the Kremlin might carry out more land grabs in pro-Russian eastern Ukraine, adding urgency to Western efforts to defuse the crisis. U.S. Secretary of State John Kerry was heading to Kyiv in an expression of support for Ukraine’s sovereignty, and the EU threatened a raft of punitive measures as it called an emergency summit on Ukraine for Thursday.

A government spokesman says Prime Minister Stephen Harper talked with his Ukrainian counterpart today by telephone.

Harper spoke to Arseniy Yatsenyuk from Toronto says a Twitter posting by Jason MacDonald, Harper’s director of communications.