Colombo- A top UN official met with Sri Lanka's President Mahinda Rajapaksa Friday to discuss humanitarian issues related to civilians trapped in a rebel-held area in the north-eastern part of the country, officials said.

Vijay Nambiar, chief of staff of UN Secretary General Ban Ki-moon, met Rajapaksa as local and international concern was growing about the safety of the civilians trapped in the war zone in Mullativu, 395 kilometres north of the capital.

Details of the meeting were not available immediately.

The meeting came as the US government expressed concern about the civilians in the area and called on both sides of the conflict - the Sri Lankan government and Tamil rebels - to stop hostilities until the civilians in the conflict area are safely evacuated.

The US embassy in a statement said the government "is deeply concerned about the current danger to civilian lives and the dire humanitarian situation" created by the fighting in Mullaitivu.

"Both sides (government and rebels) must immediately return to a humanitarian pause and both must respect the right of free movement of those civilian men, women and children trapped by the fighting," the statement said.

"The United States calls upon the government of Sri Lanka to assist its Tamil citizens by halting shelling of the safe zone, permitting international monitors to ensure the safe exit of the civilians," the statement added.

The US statement said there were some 140,000 civilians trapped in the area, but the Sri Lankan government claims the figure is around 50,000.

There is no independent confirmation about the situation in the area, where journalists and NGO are not allowed access.

Military officials say they are on the final phase of a major offensive to crush the rebels in the area, but the presence of the civilians was delaying the clearing operations.

The rebels claim that the security forces have been firing into the areas where civilians have been taking shelter which have been designation "no fire zones" by the Sri Lankan government.

Food and medicine supplies to the area have not been regular due to the fighting.(dpa)