Ukraine on Monday ordered its troops to withdraw from Crimea following the flashpoint peninsula's seizure and annexation by Russia in response to the fall in Kiev of a pro-Kremlin regime.
The dramatic announcement came just as world leaders gathered in The Hague for a nuclear security summit dominated by concern over the worst East-West crisis since the Cold War.
A top commander in NATO had warned on Sunday that the Western military alliance was carefully watching massive Russian troop formations on the eastern border of Ukraine that could theoretically make a push across the vast ex-Soviet country at any point.
Ukraine's acting president Oleksandr Turchynov told top lawmakers that both servicemen and their families would now be relocated on the mainland.
"The national security and defence council has reached a decision, under instructions from the defence ministry, to conduct a redeployment of military units stationed in the Autonomous Republic of Crimea," Turchynov said in nationally televised remarks.
"The cabinet of ministers has instructions to resettle the families of soldiers as well as everyone else who today is forced to leave their homes under the pressure and aggression of the Russian army's occupying forces."
Crimea's pro-Kremlin deputy premier Rustam Temirgaliyev told Russia's RIA Novosti state news agency that "all Ukrainian soldiers have either switched to the Russian side or are leaving the territory of the Crimea."
Turchynov's announcement came less than a month after Russian President Vladimir Putin won authorisation to use force in response to the February 22 ouster of pro-Kremlin president Viktor Yanukovych regime by more nationalist leaders who are seeking a closer alliance with Europe.
Ukraine's heavily outnumbered forces had been steadily losing ground on the Black Sea peninsula and saw their main airbase outside the regional capital Simferopol stormed by Russian troops on Saturday.
The assault by Russian forces and pro-Kremlin militias continued on Monday with the fall of a Ukrainian naval base in the east Crimean port of Feodosia.
Turchynov praised his country's soldiers despite heavy criticism by some Crimean commanders of confusion and indecision reigning among the army and naval brass in Kiev over the past month.
"Despite the enormous losses, the Ukrainian soldiers in Crimea did their duty," said Turchynov.
"Most importantly, they gave the Ukrainian armed forces the opportunity to prepare their defences, to put the military on full combat alert, and to launch a partial mobilisation."
Several top officials had warned on Sunday that they feared an attack by Kremlin forces on the heavily Russified regions of southeastern Ukraine was now imminent.
- Commando-style operation -
Russia's latest surprise assault in Crimea came in the pre-dawn hours on Monday and involved both armoured personnel carriers and stun grenades.
The Ukrainian defence ministry said Russian paratroopers were lowered onto the Feodosia naval base from four helicopters in a commando-style operation in which guns were fired in the air and stun grenades strewn across the facility in intimidation.
Several military trucks were seen leaving the base less than two hours later with some Ukrainian marines whose hands had been tied.
The Ukrainian statement said two commanders were injured when they were kicked in the face before being taken away in a helicopter in an unknown direction.
An AFP reporter saw the blue-and-yellow Ukrainian flag above the base taken down and replaced with Russia's tricolour.
The ministry later said that between "60 to 80 marines were captured" in violation of a prior agreement with the Russians.
The base in Feodosiya housed Ukraine's only marine battalion. The country's marine union said it was home to an elite unit that was part of the navy.
News agencies in Moscow cited the Russian defence ministry as saying that 189 Ukrainian military facilities and bases in Crimea were flying the Russian flag by Saturday.