Ukraine's acting president warned Crimea's Kremlin-backed leaders on Wednesday they had only three hours to release the captured head of the splintered ex-Soviet country's navy or face "an adequate response".
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.
Dozens of despondent Ukrainian soldiers -- one of them in tears -- filed out of the 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 on suspicion of "ordering Ukrainian military units... to open fire on peaceful civilians".
Gayduk's capture delivered a huge blow to efforts by the new team of untested pro-Western leaders in Kiev to impose some authority in their crisis-hit country in the face of an increasingly assertive Kremlin.
Ukraine's acting president Oleksandr Turchynov scheduled an urgent security meeting and issued a statement around 6:00 pm giving the Crimean authorities until 9:00 pm (1900 GMT) to release the commander and other "hostages".
"Unless Admiral Gayduk and all the other hostages -- both military and civilian ones -- are released, the authorities will carry out an adequate response... of a technical and technological nature."
Defence ministry officials earlier said that Russian forces had seized a second military base in Crimea's western port town of Novoozerne after using a tractor to ram open its main gate.
An AFP reporter saw about 50 Ukrainian servicemen file out of the base under the watchful eye of Russian soldiers while pro-Moscow militias lowered the Ukrainian flag.
The Ukrainian government's loss of control of the heavily Russified region was underscored further when acting defence minister Igor Tenyukh was forced to cancel a visit aimed launching Crimean mediation talks when the local authorities threatened to turn back his plane.
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.
Russia's Constitutional Court ruled unanimously on Wednesday that the "treaty complies with the Russian constitution" after a disputed Sunday referendum in Crimea showed nearly 97 percent supporting a shift from Ukrainian to Kremlin rule. Kiev and the West have dismissed the referendum as illegal.
Putin's hugely controversial treaty signing came less than a month after the ouster in Kiev of pro-Kremlin president Viktor Yanukovych by leaders who spearheaded three months of deadly protests aimed at pulling Ukraine out of the Kremlin's orbit.
The Russian leader responded by winning the right to use force against his ex-Soviet neighbour and then employing the help of local militias to seize Crimea -- a region the size of Belgium that is home to two million people as well as Russia's Black Sea Fleet.
The explosive security crisis on the EU's eastern frontier now threatens to reopen a diplomatic and ideological chasm between Russia and Western powers not seen since the tension-fraught decades preceding the 1989 fall of the Berlin Wall.
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.
Russian Foreign Minister Sergei Lavrov had already warned his US counterpart John Kerry on Tuesday that the travel bans and asset freezes unveiled by the European Union and Washington on Monday were "absolutely unacceptable and will not be left without consequences."
But Russia's tough talk appears to be pushing the new leaders of Ukraine ever further away from the Kremlin.
Ukrainian foreign ministry spokesman Yevgen Perebiynis said Kiev "reserves the right to review" its membership in the Moscow-led Commonwealth of Independent States (CIS) alliance that replaced the Soviet Union and now groups 11 states.
The greatest fear facing Kiev's new leaders and the West is that Putin will push huge forces massed along the Ukrainian border into the Russian-speaking southeastern swathes of the country in a self-professed effort to "protect" compatriots he claims are coming under attack from violent ultra-nationalists.
"We are not speaking about military actions in the eastern regions of Ukraine," Putin's spokesman Dmitry Peskov told the BBC.
"But Russia will do whatever is possible... to protect and to extend a hand of help to Russians living in eastern regions of Ukraine."
Putin signed the Crimea treaty -- recognised by no nation besides Russia -- after stressing the move was done "without firing a single shot and with no loss of life."
But the first bloodshed came only hours later when a group of gunmen wearing masks but no military insignia stormed a Ukrainian military centre in the main city of Simferopol.
The Ukrainian defence ministry said one of its soldiers died from a neck wound and another suffered various injuries.
The pro-Russia Crimean police said a member of the local militias had also been killed. A spokeswoman blamed both casualties on shooting by unidentified assailants from a nearby location.
The violence prompted the Ukrainian defence ministry to authorise its soldiers in Crimea to open fire in self-defence for the first time.
Ukraine had previously forbidden its troops from shooting -- in some cases forcing them to stand guard at their bases with empty rifles -- to avoid provoking a fully-fledged Russian offensive.