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British Prime Minister David Cameron, German Chancellor Angela Merkel and French President Francoise Hollande warned of repercussions for Russia over its "completely unacceptable" actions in Ukraine.
Cameron spoke separately to both leaders about the escalating crisis, and agreed that a joint response was needed, according to a spokesman from Cameron's office.
"In both calls, the leaders agreed that the international community should speak with one voice and send a clear message to Russia that its actions in Ukraine were completely unacceptable," said the statement yesterday.
A special European Council has been called for Thursday, and the three leaders agreed "it would be important to be clear about the costs and consequences for the Russian government of continuing to violate Ukraine's sovereignty," said the spokesman.
The trio vowed to support the Ukrainian people and government and also suggested a major role for the United Nations and other international organisations in resolving the crisis.
Cameron earlier earned that Russia will face diplomatic, political, and economic pressure in response to its violation of Ukraine's sovereignty.
Cameron, who chaired a meeting of British military and security chiefs in London to discuss the crisis, said the world needed to send a "clear message" to Moscow.
He said "What we want to see is a de-escalation rather than a continuation down the path that the Russian government has taken, violating the sovereignty and territorial integrity of another country.
"So we shall have to bring to bear diplomatic, political, economic and other pressures in order to make this point.
"That is the very clear message the whole world needs to send to the Russian government."
Cameron was speaking after Ukraine's military said Russia had issued an ultimatum to Ukrainian forces to surrender in the Crimea or face an all-out assault on the Black Sea peninsula.
The Russian Black Sea fleet dismissed the claims as "nonsense", but Cameron said the reports were "deeply concerning", adding that "we all want to see stability and countries obeying the rule of law".
British Foreign Secretary William Hague warned Russia yesterday of "consequences and costs" as he met Ukraine's new Western-backed leaders in Kiev.
"This cannot be the way in the 21st century to conduct international affairs," Hague said.
"It is not an acceptable way to behave and there will be consequences and costs," he added.
