KIEV (AFP) - Ukraine announced plans Wednesday to drop out of a key post-Soviet alliance and slap entry visas on Russians while also preparing for a possible Crimean withdrawal following the Kremlin's absorption of the peninsula.
Kiev's first firm response to Russia's claim to the strategic Black Sea peninsula came as a deadline expired on an ultimatum set by the acting president for Crimea's separatist leaders to release the captured head of the Ukrainian navy or face "an adequate response".
The spiralling crisis prompted the White House to warn Russia it was "creating a dangerous situation" and the NATO commander to call the Kremlin's seizure of Crimea "the gravest threat to European security and stability since the end of the Cold War".
Germany for its part said it was suspending a major arms deal with Moscow - a signal that Washington's EU allies were willing to take more serious punitive steps against the Kremlin despite their heavy dependence on Russian energy supplies.
Even US Federal Reserve Chair Janet Yellen said a further escalation "would certainly be something that would be on our radar screen" as the central bank charts a course for the world's largest economy.
But Moscow appeared ready to up the diplomatic stakes by warning Washington it was preparing a "wide range" of countermeasures should the United States follow through on threats to impose broad economic sanctions against Russia.
Pro-Russian forces had earlier seized two Crimean navy bases and detained Ukraine's naval chief as Moscow tightened its grip on the flashpoint peninsula despite Western warnings that its "annexation" would not go unpunished.
Ukraine's National Security and Defence Council chief Andriy Parubiy said Kiev had decided to withdraw from the Moscow-led Commonwealth of Independent States (CIS) alliance that replaced the Soviet Union and to slap visas on Russians who sought to enter the country in response to the Kremlin's Crimean claim.
A defiant President Vladimir Putin had brushed aside global indignation and Western sanctions on Tuesday to sign a treaty absorbing Crimea and expanding Russia's borders for the first time since World War II.
British Prime Minister David Cameron said on Wednesday that next week's meeting of leaders from the Group of Seven (G7) most developed economies must discuss Russia's permanent expulsion from the wider G8 political grouping to which Moscow was accepted in 1998 as its reward for pursuing a democratic course.
United Nations chief Ban Ki-moon will meet with Putin in Moscow on Thursday before holding talks with Ukraine's interim leaders in Kiev on Friday to encourage a peaceful resolution of the crisis.
Russia's Deputy Foreign Minister Sergei Ryabkov warned Washington on Wednesday that Moscow was preparing an entire series of "asymmetrical measures" should the United States hit his country with economic trade restriction or other more severe steps.
Ryabkov said these measures covered "a number of areas of dialogue... that are important to the Americans" and hinted that Russia could "raise the stakes" in the ongoing Iranian nuclear talks.