Armed servicemen stand near a Ukrainian border guard post in the Crimean town of Balaclava.
Russian forces have given Ukrainian soldiers an ultimatum to surrender their positions in Crimea or face an assault, a Ukrainian defence ministry spokesman told AFP.
"The ultimatum is to recognise the new Crimean authorities, lay down our weapons and leave, or be ready for an assault," said Vladyslav Seleznyov, the regional ministry spokesman for the Crimea. He said base commanders had informed the ministry of "different times" for the ultimatum to expire.