Cameron, who spoke separately to both the leaders about the escalating crisis, agreed that a joint response was needed, stated a spokesman from Cameron's office. (Agencies)
"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 on Monday.
A special European Council has been called for Thursday and the three leaders have agreed to it.
"It is important to be clear about the costs and consequences for the Russian government if it continues to violate Ukraine's sovereignty," said the spokesman.
The trio vowed to support the Ukrainian people and government. They also suggested a major role for United Nations and other international organisations in resolving the crisis.
Cameron earlier said that Russia will face diplomatic, political and economic pressure in response to its violation of Ukraine’s sovereignty.
“The world needed to send a ‘clear message’ to Moscow,” he added.
Cameron further 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,” he added.
The British Prime Minister was speaking after Ukraine’s military said Russia had issued an ultimatum to Ukrainian forces to surrender in Crimea or face an all-out assault on the Black Sea peninsula. The Russian Black Sea fleet dismissed the claims as ‘nonsense’.
Cameron on his part said that the reports were ‘deeply concerning’.
“We all want to see stability and countries obeying the rule of law,” he added.
British Foreign Secretary William Hague warned Russia on Monday 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 serious repercussion,” he added.
Cameron, who spoke separately to both the leaders about the escalating crisis, agreed that a joint response was needed, stated a spokesman from Cameron's office.