Armed men believed to be Russian seized a Ukrainian military base in Crimea 0803141.

“The base is now back in full control of the Ukrainian armed forces,” said a spokesman for the Ministry of Defense, asking to remain anonymous.

A Ukranian military official, Vlad Seleznyov, told Reuters that men took over the base - where some 100 Ukrainian troops are stationed - without any shooting, and that no one had been hurt.

All of Ukraine's military bases in the Crimea that have not yet surrendered to the pro-Russian side are understood to be under siege.

The Pentagon on Friday estimated as many as 20,000 Russian troops may be in Crimea but acknowledged its information was imperfect, as U.S. Defense Secretary Chuck Hagel praised the restraint of Ukrainian forces.

Serhiy Astakhov, an aide to the border guards' commander, said 30,000 Russian soldiers were now in Crimea, compared with the 11,000 permanently based with the Russian Black Sea fleet in the port of Sevastopol before the crisis.

The United State has also moved to reassure NATO allies, sending six more F-15 fighter jets this week to NATO's policing mission over the Baltic states of Estonia, Latvia and Lithuania.