Ukraine's acting president has described his country's confrontation with Russia as a "war" as he sent in troops to drive out suspected Russian soldiers from the east of the country, after fatal clashes and claims that thousands more are massed near the border.
Oleksandr Turchynov made a televised address in which he told the nation that it was at war with Russia, after pro-Russian forces took over government buildings and a Ukrainian security officer was killed by militia.
“The blood of Ukrainian heroes has been shed in a war that the Russian Federation is waging against Ukraine,” Mr Turchynov said, according to The Times (£). “The aggressor has not stopped and is continuing to sow disorder in the east of the country.”
On Sunday, the British Ambassador to the UN has accused Russia of waging "a well-organised campaign to destabilise Ukraine" at an emergency meeting of the United Nations Security Council.
Moscow has denied it is behind the co-ordinated operation by gangs wearing uniforms without insignia, but Britain said Russia would be assumed to be complicit unless it condemned the actions.
Speaking at the security council meeting in New York, the UK's ambassador to the UN, Sir Mark Lyall Grant, warned of a "dangerous escalation of an already dangerous situation" about which the UK is "gravely concerned".
Satellite images show between 35,000 and 40,000 Russian troops in the vicinity of the border with Ukraine, along with combat aircraft, tanks and artillery, in addition to the 25,000 Russian troops already illegally in Crimea, he said.
While the Kremlin denied it was behind the attacks in eastern Ukraine, Sir Mark said the events "point clearly to Russia once again interfering directly in the internal affairs of Ukraine using manufactured pretexts and military force", calling recent events in eastern cities "a well-orchestrated campaign designed to destabilise the country".
Urging Russia to back down and air its grievances through dialogue, Sir Mark said: "Russia's aggressive stance is a clear threat to the sovereignty, independence and territorial integrity of Ukraine.
"The United Kingdom calls on Russia to immediately redeploy its troops away from the border, to cease all activity designed to heighten tensions and sow discord, and to revoke the mandate given by the Federation Council on 1 March to use military force on Ukrainian soil."
Foreign Secretary William Hague will meet European counterparts in Luxembourg later on Monday to discuss the developments.
There has been unrest in the east of the country, including Donetsk, where ethnic Russians fear they will suffer at the hands of the West-facing Ukrainian government.
Ukraine's interim president Oleksandr Turchynov announced on Sunday he would send in the army to resist attacks, stop Russian forces moving in and prevent a repeat of the situation in Crimea.
Russian ambassador Vitaly Churkin denied Moscow was to blame for the violence, instead accusing Ukraine of using radical neo-Nazi forces to destabilise its eastern region.
But U.S. UN ambassador Samantha Power accused Russian of "spreading fiction", saying: "These are not protests, these are professional military operations."
A Foreign Office spokesman said: "The latest occupations by armed groups of government buildings in towns in eastern Ukraine are a further dangerous escalation of an already dangerous situation.
"Assumptions that Russia is complicit are inevitable as long as Moscow does not publicly distance itself from these latest lawless actions. Russia must desist from steps which destabilise Ukraine and undermine the possibility of Contact Group talks.
"European foreign ministers will meet on Monday to discuss how to respond to the latest events."