Ukraine has ordered its armed forces to withdraw from Crimea following the capture of its last military base on the peninsula by Russia.
Interim President Oleksander Turchinov told parliament the decision to withdraw the troops was taken in the face of threats to service personnel and their families.
A Ukrainian defence official has told the BBC that every Crimean military base is now under Russian control.
The move completes Russia's annexation of Crimea, following a referendum on 16 March.
Mr Turchinov said military units in Crimea will be redeployed and their families evacuated.
A BBC correspondent said the order to withdraw can be regarded as a de facto military surrender.
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.
G7 leaders are holding an emergency meeting at the Dutch Prime Minister's residence to co-ordinate a response to Russia's actions in Ukraine.
Before the talks, US President Barack Obama said Europe and America were united in their support of the Ukrainian government and its people.
Mr Obama said the United States and Europe were also "united in imposing a cost on Russia for its actions (in Ukraine) so far".
So far, the US has imposed sanctions on 27 individuals and one bank, while the European Union has penalised 33 individuals.
British prime minister David Cameron said it's absolutely clear there will not be a summit of the G8 nations this year in Russia.
Preparations for the meeting in Sochi have been suspended since Russia's annexation of Crimea.
Ukraine situation takes over summit - Key
The crisis in Ukraine has dominated talks on the first day of the Nuclear Security Summit in The Hague.
New Zealand Prime Minister John Key said aside from an emergency meeting of G7 leaders, other heads of state have been discussing their response to the situation and how that might play out.
Mr Key said his bilateral meeting with German chancellor Angela Merkel quickly moved to the Ukraine crisis.
He said she can see how destabilising it could be for Europe, with 18 of the 28 European countries taking half their gas from Russia.
Mr Key told her it is about putting broader interests ahead of any specific economic interests.
Radio New Zealand's reporter travelling with Mr Key says on the floor of the summit a Ukrainian representative spoke only about the situation in his country rather than about nuclear security.