Crimea, Ukraine (BBN)-Ukraine's interim President Olexander Turchynov says he has ordered the withdrawal of armed forces from Crimea, BBC reports.
The decision was taken because of Russian threats to the lives of military staff and their families, the president announced.
A Ukrainian defence official has told the BBC that every Crimean military base is now under Russian control.
Earlier this month, Russia annexed Crimea after a referendum which Ukraine and the West considered illegal, the report added.
The G7 group of industrialised countries is to consider a collective response to the crisis during talks in The Hague.
G7 leaders are meeting on the sidelines of a long-planned summit on global threats to nuclear security.
Speaking ahead of the talks, US President Barack Obama said Europe and America were united in their support of the Ukrainian government and its people.
Alongside Dutch Prime Minister Mark Rutte, Obama said the US and Europe were also "united in imposing a cost on Russia for its actions (in Ukraine) so far".
'PRESSURE AND AGGRESSION'
Interim President Turchynov announced the withdrawal of Ukrainian troops from Crimea in a nationally televised statement.
"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," he told the Ukrainian parliament.
"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."
Just two naval ships and a single police base are still flying the Ukrainian flag, a senior official at the Ukrainian defence ministry has told the BBC.
Russian defence officials earlier said that the tricolour of Russia had been hoisted at 189 Ukrainian military units and facilities in Crimea.
Earlier on Monday, Russian troops captured the naval base at Feodosia, the third such takeover in 48 hours.
Defence spokesman Vladislav Seleznyov said the Russians had attacked the base from two directions using armoured personnel carriers and stun grenades.
An official withdrawal from the base was due to start at 15:00 (13:00 GMT), Seleznyov added.
The BBC's Mark Lowen in Crimea says the order to withdraw is hugely significant and can be regarded as a de facto military surrender, although Kiev would never admit it.
Our correspondent says this is now the end of the battle for Ukraine, with its military presence in Crimea almost completely crushed.
Nato's military commander in Europe warned on Sunday that Russian forces on Ukraine's eastern borders were capable of mounting an operation all the way to Moldova.
Moscow's annexation of Crimea on 16 March came after protesters overthrew pro-Kremlin President Viktor Yanukovych in February.
Russia said it had acted to protect its "compatriots" in Crimea from "fascists" moving in from mainland Ukraine.
The US and EU have responded with a series of sanctions targeting those individuals including senior officials whom they accuse of involvement in Crimea's annexation.
BBN/ANS/AD/24Mar14-9:07 pm (BST)