1 March 2014
Last updated at 14:31 ET
The Russian parliament has backed President Putin’s request to use troops in Ukraine
Prime Minister David Cameron has said there can be “no excuse” for outside military intervention in Ukraine.
He was speaking after the Russian parliament approved President Putin’s request to deploy troops there.
Mr Cameron said everyone “must work to lower, not escalate, tensions. The world is watching.”
Foreign Secretary William Hague said he had passed this message to his Russian counterpart, Sergei Lavrov, in a phone call earlier on Saturday.
Mr Hague said Ukraine’s sovereignty over the Crimea region should be respected and Britain opposed Russian moves to use troops there.
He added: “We have now summoned the Russian ambassador to the Foreign Office to register our deep concerns.”
Shadow foreign secretary Douglas Alexander said: “The vote in the Russian parliament to approve Russian forces being mobilised is deeply concerning and represents a potentially grave escalation of the situation in Ukraine.
“All efforts must be made to try to secure a diplomatic resolution to the crisis.”
The Foreign Office advises against all travel to Crimea and says British nationals there “should leave now by any practical means”.
On Friday, the airport at Simferopol in Crimea was reported to have been seized by armed men.
The Foreign Office said the airport was reported to be operating as usual but it did not advise British nationals to try to leave from there until the situation had become clearer.
It said it was not able to provide consular services to anyone who chose to remain in Crimea.
It said anyone who does stay in the area should “keep a low profile, avoid areas of protest or stand-off and stay indoors where possible”.
The British embassy in the Ukrainian capital, Kiev, is now open to the public by appointment only.