Russia has given Ukrainian forces in Crimea a deadline of 3am on Tuesday to surrender or face military action after troops seized key strategic sites in the peninsula.
The ultimatum came from Alexander Vitko, commander of Russia’s Black Sea Fleet, which has a base in Crimea where Russian forces are now in control.
According to Russia’s Interfax agency, it reads: “If they do not surrender before 5am (3am UK time) tomorrow, a real assault will be started against units and divisions of the armed forces across Crimea.”
It came as Russian President Vladimir Putin watched tanks and armoured vehicles taking part in military exercises at a training ground in north-west Russia.
Mr Putin attended the final day of war games he ordered on February 26 to test the combat-readiness of his armed forces in western and central parts of Russia, regions adjacent to Ukraine, a spokesman said.
Russian Prime Minister Dmitry Medvedev discussed the escalating crisis in Ukraine with US Vice President Joe Biden by telephone on Monday.
Mr Medvedev “declared that it is necessary to protect the interests of all Ukrainian citizens, including residents of Crimea, and citizens of Russia who are located in Ukraine,” according to Interfax.
He added that Russia would press ahead with plans to build a bridge linking Russia directly with the Crimea region – providing a vital transport link to the Black Sea peninsula.
Mr Medvedev told deputies the two countries had signed “documents related to a project for construction of a transport corridor across the Kerch Strait” in December, when now-ousted Ukrainian president Viktor Yanukovych was still in power.
The Russian foreign ministry said Nato’s criticism of its actions in Crimea “will not help stabilise” the situation in Ukraine.
Hundreds of Russian soldiers have surrounded a military base in Crimea, preventing Ukrainian soldiers from going in or out.
The convoy blockading the site, near the Crimean capital Simferopol, includes at least 17 military vehicles.
Russian troops have also reportedly taken control of a ferry terminal in the city of Kerch, on the eastern tip of Crimea, which has a majority Russian-speaking population.
Ukraine’s defence ministry said two Russian fighter jets violated the country’s air space in the Black Sea on Sunday night and that it had scrambled an interceptor aircraft to prevent the “provocative actions”.
Elsewhere, pro-Russian protesters have taken over a floor of the regional government building in Donetsk, say reports. The 11-storey building has been flying the Russian flag for the last three days.
The crisis has had a huge effect on global stock markets, with Moscow’s stock exchange plunging more than 10% on Monday.
Russia’s central bank raised its rate to 7% from 5.5% as the ruble hit an historic low against the dollar and the euro.