The Sri Lankan president today rejected calls for a ceasefire even as leaked images from the UN appeared to confirm that the military shelled the tiny coastal area of Mulattivu where civilians were sheltering in a no-fire zone.

The UN satellite images, the most recent of which were taken on 19 April, indicate heavy destruction in and around an area covering 3.8 square miles, where at least 50,000 people are trapped.

The latest blow to the Sri Lankan government's credibility came from Unosat, which provides satellite imagery for the UN and relief agencies. Its report dated 26 April said there were indications of "building destruction and damages resulting from shelling and possible air strikes". The report also mentions attacks outside the area.

"Outside the CSZ [civilian safe zone], immediately to the west and south there are continuing indications of sustained shelling and possible air strikes, with the major concentration of newly detected building destruction and hundreds of impact craters located in the eastern sections of Puthukkudiyiruppu especially centred on the main road leading east into the CSZ," the report said.

Unosat mentioned the movement of civilians within the safe haven as they sought to get away from military attacks. "Over 5,000 IDPs [internally displaced persons] were relocated within the CSZ during April under pressure from increased shelling and military operations along the western sections of the CSZ," it said.

Sri Lanka this week pledged to stop using heavy weapons in an attempt to prevent civilian casualties. But there have been persistent reports of continued shelling and air strikes. The Associated Press today reported that the military fired 100 shells into the area starting last night and continuing until this morning.

A government health official who witnessed the barrage told the Associated Press that one shell hit the top of a coconut tree and exploded, scattering shrapnel over civilians below and killing 10. Reports of artillery fire have undercut government declarations that the area is safe for civilians.

At the same time, US officials say there are credible reports that Tamil Tigers are using civilians as human shields and have in some cases shot at civilians trying to leave the area.

International concern over the plight of Tamil civilians trapped in the war zone has grown in recent days following a UN report that nearly 6,500 civilians were killed in the last three months of an offensive that has decimated the Tamil Tiger rebels.

The British foreign secretary, David Miliband, was reportedly involved in a heated exchange with a Sri Lankan minister over the safety of civilians in the area held by the Tamil Tigers. Miliband interrupted the Sri Lankan defence secretary, Gotabaya Rajapaksa, as he was describing how the army had rescued 200,000 civilians from the rebels to say that Britain had credible information that civilians were being harmed by the military's attacks.

The Sri Lankan president, Mahinda Rajapaksa, has rejected calls for a ceasefire and today struck a triumphalist note. "We fearlessly stood up to a brand of terrorism that the entire world believed was invincible," he said in a May Day address.

The Tamil Tigers have been fighting since 1983 for a state for minority Tamils in the north and east after decades of marginalisation by governments dominated by the Sinhalese majority.

They appear to be on the verge of defeat as the latest military offensive has cornered them in a tiny strip of the north-eastern coast.

The government and aid groups have been struggling to cope with more than 120,000 civilians who fled the war zone last week, overwhelming displacement camps.

Amid the fighting, the International Monetary Fund is coming under pressure from the US to postpone a $1.9bn (£1.3bn) emergency loan to Sri Lanka, but other UN security council members are resisting any delay.