NATO called on Russia on Sunday to withdraw its forces from Ukraine to their bases, as Ukrainian officials said they were prepared to defend themselves and US Secretary of State John Kerry prepared a mission to Kiev.

The German government said Russian President Vladimir Putin accepted a proposal by Chancellor Angela Merkel for a fact-finding mission and a "contact group" on Ukraine that could be led by the Organization for Security and Cooperation in Europe.



Meanwhile, in a sign of growing international pressure, Secretary of State John Kerry threatened Russia with sanctions and other economic consequences if it did not back down. He will visit Ukraine's capital Tuesday, a senior administration official said.



Ukraine's security council ordered the general staff to immediately put all armed forces on highest alert, the council's secretary Andriy Parubiy announced. The Defence Ministry was ordered to conduct the call-up, potentially of all men up to 40 in a country that still has universal male conscription.

The country also withdrew its Coast Guard vessels from two Crimean ports, in what was taken as a sign that Russia was nearing the completion of its seizure of the peninsula. But hours later, the Ukrainian government said it had fired its Navy chief and opened a treason case against him for failing to defend his Sevastopol headquarters. He was reported to have appeared on television pledging allegiance to pro-Russian authorities in Crimea.

(Read more: Obama challenges Putin on 'clear violation')



"If aggravation is going in that way, when the Russian troops are enlarging their quantity with every coming hour, naturally we will ask for military support and other kind of support," Ukraine's UN ambassador Yuriy Sergeyev told CNN.



Russian forces who have already bloodlessly seized Crimea - an isolated Black Sea peninsula where most of the population are ethnic Russian and Moscow has a naval base - tried to disarm the small Ukrainian contingents there on Sunday. Some Ukrainian commanders refused to give up weapons and bases were surrounded.

A senior U.S. administration official said late Sunday that Russia now has complete operational control of the Crimea peninsula, with 6,000 air and naval forces in the region.

Associated Press journalists reported Sunday that hundreds of unidentified gunmen have arrived outside Ukraine's infantry base in Privolnoye in its Crimea region.

The convoy included at least 13 troop vehicles each containing 30 soldiers and four armored vehicles with mounted machine guns, the AP journalists reported. The vehicles — which have Russian license plates — have surrounded the base and are blocking Ukrainian soldiers from entering or leaving it.

Ukrainian soldiers, with clips in their weapons, have positioned a tank at the gate.