Ukraine has announced plans 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 on Wednesday 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.
Read: All you need to know about Ukraine
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 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.
But Moscow appeared ready to up the diplomatic stakes, warning Washington it was preparing a "wide range" of countermeasures should the United States follow through on threats to impose broad economic sanctions.
Read: Obama says Russia has violated international law in Ukraine
Pro-Russian forces 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.
Dozens of despondent Ukrainian soldiers -- one of them in tears -- filed out of Ukraine's main navy headquarters in the historic Black Sea port city of Sevastopol after it was stormed by hundreds of pro-Kremlin protesters and masked Russian troops.
The local prosecutor's office said Ukraine's navy commander Sergiy Gayduk -- appointed after his predecessor switched allegiance in favour of Crimea's pro-Kremlin authorities at the start of the month -- had been detained.
Russian Defence Minister Sergei Shoigu later urged Crimea's pro-Russian leaders to free Gayduk, but only after the expiry of a 0030 IST deadline set by Ukraine's acting president Oleksandr Turchynov for the Crimean authorities to release the commander.
Read: Putin has chosen path to Russia's isolation: Britain
His capture was a blow to efforts by a new team of untested pro-Western leaders in Kiev to impose some authority in the face of an increasingly assertive Kremlin.
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.
A Ukrainian soldier stands guard at a check point at the border between Ukraine and Crimea near the Salkovo village near Kherson. (AFP Photo)
Parubiy added that Ukraine was also developing a contingency plan to withdraw Crimean servicemen and their family members "so that they could be quickly and efficiently moved to mainland Ukraine".
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.
Read: How seriously does Russia's Vladimir Putin take US threats?
Crimea annexation: In Putin vs West, India can't pick a side
Welcome to Republic of Crimea, the world's newest statelet