UN aid agencies are still waiting for visas for disaster response teams to enter Burma some four days after a devastating cyclone struck the reclusive country, officials said today.

"For the moment we have a five-member disaster assessment team on standby in Bangkok waiting for their visas, we expect them to be dispatched in the next hours,'' Elisabeth Byrs, spokeswoman for the UN's Office for the Coordination of Humanitarian Affairs, said.

The UN children's fund and the International Federation of Red Cross and Red Crescent Societies (IFRC) said they were also waiting for visas in order to help distribute urgently needed humanitarian supplies such as tents, water purification tablets and mosquito nets.

"The situation is extremely serious, we are very, very worried,'' Ms Byrs said.

"One can say that the damage that has hit this region, especially the delta region, is at least equivalent to what we saw in the areas worst hit by the tsunami of 2004 which devastated Southeast Asia," she added.

Burma's state media estimates that at least 15,000 people died after Cyclone Nargis struck on May 2 and 3.

The country's ruling generals, who normally screen foreign aid closely and keep a tight rein on international relief organisations, made a rare appeal for help from abroad after the storm barrelled into the country overnight Friday.

But a government minister said today that foreign aid teams wanting to come to the military-ruled nation after the devastating cyclone would have to negotiate with the regime to be granted access.

"For expert teams from overseas to come here, they have to negotiate with the foreign ministry and our senior authorities,'' Maung Maung Swe, the minister for social welfare, said.