Foreign Secretary David Miliband and US Secretary of State Hillary Clinton have expressed their "profound concern" over the humanitarian crisis in Sri Lanka.
They issued a joint statement after a meeting in Washington to draw attention to the plight of residents caught up in the fighting in the country’s war zone.
Hundreds of civilians have died in days of shelling in the north east region, with some putting the death toll at more than 1,000.
Before the meeting with Mrs Clinton, Mr Miliband described conditions in the war zone as "hell".
Around 50,000 people are trapped in an area of 3km sq in north east Sri Lanka and the military offensive has seen scores of children killed.
Rebels blame the government for the deaths, while the military has accused the Tamil Tigers of shelling their own territory to win international sympathy.
In the joint statement, the UK and US called for both sides to end the fighting immediately and allow for the safe evacuation of civilians.
It added: "The Liberation Tigers of Tamil Eelam must lay down their arms and allow civilians free passage out of the conflict zone.
"The government of Sri Lanka must abide by its commitment of April 27 to end major combat operations and the use of heavy weapons."
The statement called for a political solution to the conflict that reconciles all Sri Lankans and "establishes a meaningful role for Tamil and other minorities".
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