Mr Miliband and Mrs Clinton issued a statement after talks in Washington
The US and UK have urged Sri Lanka's government and Tamil Tiger rebels to stop fighting "immediately" and allow an evacuation of trapped civilians.
US Secretary of State Hillary Clinton and her UK counterpart David Miliband also expressed alarm at the large number of reported civilian casualties.
The top UN aid official said the situation in the conflict zone in north-eastern Sri Lanka was "awful".
The rebels earlier accused the army of killing 49 people in a hospital.
The Tamil Tigers said the makeshift hospital in Mullivaikal in the rebel-held enclave was hit on Tuesday morning.
The Sri Lankan government denied the army had caused civilian casualties or used heavy weapons in civilian areas, but said it had pierced rebel defences.
As it advanced south, the army also said that all voices speaking from the Tiger-held zone amounted to misinformation.
The claims are impossible to verify as reporters are banned from the area.
More than 400 people were killed and over 1,000 injured over the weekend in what the UN has described as a "bloodbath".
The UN estimates that about 50,000 civilians are trapped by the conflict, in a three-sq-km strip of land. Most of this area has been designated by the government as a safe zone which will not be attacked by air or by heavy weapons.
Mrs Clinton and Mr Miliband issued a joint statement on Sri Lanka, following their talks in Washington.
The statement urged all sides in Sri Lanka to "end hostilities immediately and allow for the safe evacuation of the tens of thousands of civilians trapped within the safe zone".
It also said London and Washington were alarmed "at the large number of reported civilian casualties over the past several days in the designated 'safe zone'".
The two top diplomats called for "a political solution that reconciles all Sri Lankans, and establishes a meaningful role for Tamil and other minorities in national political life".
Their appeal was the latest in a series of calls by the international community to try to end the fighting on the Indian Ocean island.
Aid shipment abandoned
Earlier on Tuesday, UN humanitarian co-ordinator John Holmes said intransigence by both the Sri Lankan government and the rebels had created an "absolutely awful situation".
"The LTTE [Tamil Tigers] are clearly still holding onto that population against their will, using them as human shields," he said at a news conference in Geneva.
"The government have said they are not using heavy weapons. But the evidence suggests that they are continuing to do so, at least to some extent."
Meanwhile, the International Committee of the Red Cross (ICRC) on Tuesday abandoned an attempt to deliver aid by sea to the enclave and evacuate many sick and injured civilians.
An ICRC spokeswoman said the fighting was too fierce and another attempt would be made on Wednesday. The last evacuation was on Saturday.
The Tamil Tigers have fought for an independent homeland for Sri Lanka's Tamil minority since 1983.
More than 70,000 people have been killed in the war.
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