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 are also reported to have 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”.
And reports claimed pro-Russian protesters had occupied a floor of the regional government building in Donetsk. The 11-storey building has been flying the Russian flag for the last three days.
Ukrainian prime minister Arseny Yatseniuk has insisted his country “will never give up Crimea to anyone” and urged Russian forces to withdraw.
Mr Yatseniuk said: “I was and am a supporter of a diplomatic solution to the crisis, as a conflict would destroy the foundations for stability in the whole region.”
In an interview with Sky News, Foreign Secretary William Hague said the crisis is likely to take some time to resolve.
He said: “I think we probably are looking at a long period of very active diplomacy and looking for solutions to this since there is no sign of a change in the Russian position on this.
“It’s impossible to be optimistic at the moment. We’re not in any position to be optimistic about the security situation and what is happening in the Crimea.”
The crisis has had a huge knock-on effect on global stock markets, with Moscow’s stock exchange plunging as much as 10% on Monday morning.
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.
Russian foreign minister Sergei Lavrov discussed Ukraine by telephone with his Chinese counterpart, Wang Yi, on Monday, and claimed they had “broadly coinciding points of view” on the situation there, according to a ministry statement.
Speaking at the UN Human Rights Council in Geneva later, Mr Lavrov said Russian troops were necessary in Ukraine “until the normalisation of the political situation” and dismissed threats of sanctions and boycotts.