Updated | Russia 'denies' giving Ukrainian forces in Crimea ultimatum to surrender
Russia's Black Sea Fleet has given Ukrainian forces in Crimea until 5:00 local time (03:00 GMT), Ukraine defence ministry says.
Russia's Black Sea Fleet has told Ukrainian forces in Crimea to surrender by 5 a.m. on Tuesday or face a military assault, Interfax news agency quoted a source in the Ukrainian Defence Ministry as saying.
Russia has however denied issuing such an ultimatum.
The ultimatum, Interfax said, was issued by Alexander Vitko, the fleet's commander.
The ministry did not immediately confirm the report and there was no immediate comment by the Black Sea Fleet, which has a base in Crimea, where Russian forces are in control.
"If they do not surrender before 5 a.m. tomorrow, a real assault will be started against units and divisions of the armed forces across Crimea," the agency quoted the ministry source as saying.
Ukraine said Russia was building up armoured vehicles on its side of a narrow stretch of water closest to Crimea after Putin declared at the weekend he had the right to invade his neighbor to protect Russian interests and citizens.
On the ground in Perevalnoye, half way between the Crimean capital of Simferopol and the Black Sea, hundreds of Russian troops in trucks and armoured vehicles - without national insignia on their uniforms - surrounded two military compounds, confining Ukrainian soldiers as virtual prisoners.
Ukraine called up reservists on Sunday and the United States threatened to isolate Russia economically after Putin's action provoked what Britain's foreign minister called "the biggest crisis in Europe in the twenty-first century".
European Union foreign ministers were to hold emergency talks on Ukraine on Monday but diplomats said they would not immediately match U.S. threats of sanctions against Moscow, but would focus on diplomatic efforts to prevent escalation.
The Organisation for Security and Cooperation in Europe (OSCE) said it was trying to convene an international contact group to help defuse the crisis after Germany said Chancellor Angela Merkel had convinced Putin to accept such an initiative.
Switzerland, which chairs the pan-European security body, said the contact group would support Ukraine during its transition and coordinate aid and could also discuss sending observers to monitor the rights of national minorities.