This undated photo released by pro-Tamil media reportedly shows a government air strike inside a no-fire zone and civilians fleeing
Tamil Tiger rebels fighting government forces in north-east Sri Lanka have declared a unilateral ceasefire.
Rebel spokesman Seevaratnam Puleethevan told the BBC the move was due to an "unprecedented humanitarian crisis".
Sri Lanka's defence secretary however dismissed the announcement as "a joke", insisting the rebels must surrender.
The rebels have been beaten back to a 12 sq km (5 sq m) area. The UN says some 50,000 civilians remain trapped but the army puts the number at 15,000.
The defence ministry reports capturing the village of Valayanmadam, and says 23 rebels surrendered and 200 civilians were "rescued" there on Sunday morning. The report could not be verified independently.
The Tigers' announcement came as the United Nations' top humanitarian official, John Holmes, was meeting Sri Lankan officials to call for access for aid workers to the war zone and government-run camps for thousands of displaced people.
Aid workers have been barred from the area since the fighting escalated last year and the rebels say the government is deliberately blocking food aid - a charge the Sri Lankan authorities deny.
The rebels said they were responding to calls made by the UN, EU, the governments of India and others.
From Charles Haviland in Colombo
Outlawed as a terrorist group by many countries, the Tigers are trying to show they are clearly aligned with the UN and Western countries at this point, in contrast to the Sri Lankan government which is continually rejecting the chorus of international calls for a halt in its offensive.
This is not the first time the Tigers have declared their readiness for a ceasefire in recent months. Their past offers have often been sent to key international players with an interest in the Sri Lankan situation.
The government claims the war is nearly at an end and alleges that the Tigers have used past ceasefires to regroup and re-arm.
They said the unilateral ceasefire would come into immediate effect.
Spokesman Mr Puleethevan added that the Tigers would maintain their ceasefire only if the government reciprocated.
"It is purely for humanitarian purposes and the duration will depend on the response of the Sri Lankan government," he told AFP news agency by phone from inside rebel-held territory.
Defence Secretary Gotabaya Rajapaksa told the BBC the Tigers were the "the losing side."
Saying the army was within "walking distance" of rebel-controlled areas, he called for the rebels to surrender immediately and release all civilians within the conflict zone.
Images from surveillance drones showed there were 15,000 civilians left in the war zone, he added.
Denying media reports that he had been stopping food supplies to trapped civilians, he said that he himself had asked the World Food Programme and the Red Cross to deliver relief to them.
A bitter propaganda battle is in progress, the BBC's Charles Haviland reports from the Sri Lankan capital, Colombo.
The Tigers accuse the government of bringing the threat of starvation to civilians inside the war zone.
Troops have ringed off the Tigers
The government and pro-government media accuse the Tigers of keeping Tamil civilians as hostages and murdering people trying to flee their control, including little children.
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, but that figure could now be far higher because of intensified fighting in recent weeks.
Hospitals and government-run camps for displaced people in the north-east have been flooded by people fleeing the shrinking rebel-held zone as the military closes in on the rebels.
'Very dire' situation
Speaking in Thailand on his way to Sri Lanka, Mr Holmes said the civilians caught up in the conflict were suffering not only a high casualty rate from the fighting but from a lack of access to food, clean water and medical supplies.
"The situation of these people is very dire and that's why we need to somehow find a way to stop the fighting and get them out of there so we can look after them properly," he said.
A UN document being circulated around diplomatic missions in Sri Lanka estimates that nearly 6,500 civilians have died and 14,000 have been injured since the end of January.
The White House said it was "deeply concerned about the plight of innocent civilians caught up in the conflict between the government of Sri Lanka and the Tamil Tigers and the mounting death toll".
It called on both sides to adhere to international humanitarian law and to "stop fighting immediately and allow civilians to safely leave the combat zone".
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