YANGON, Myanmar - Five helicopters working for the U.N. landed yesterday in Myanmar to help with cyclone relief efforts, as problems continued to dog efforts to help victims of the May 2-3 storm.

The five chartered helicopters - two from South Africa, and three from Uganda - were transported to Bangkok, Thailand, more than a week ago and flew into Myanmar's biggest city, Yangon, said U.N. World Food Program spokesman Paul Risley. They join one that began ferrying supplies on WFP's behalf to the hard-hit Irrawaddy delta on Monday.

"This is a big boost in the U.N. cyclone response capacity," said Risley. "The helicopters will multiply the work of the first helicopter, which has been shuttling food and humanitarian assistance."

He said a final group of four helicopters for WFP use, now in Bangkok, would likely fly to Myanmar in the coming week.

The relief effort, however, still faces a myriad of problems, among them a severe shortage of housing materials that could leave hundreds of thousands of survivors exposed to heavy rains as the monsoon season begins, aid agencies said.

The United Nations and the International Federation of Red Cross and Red Crescent Societies warned there was an "urgent need" for tarpaulins to provide the estimated 1.5 million homeless survivors with temporary shelter.

"Exposure to the elements five weeks after a disaster of this magnitude has to be a major concern," said John Sparrow, a spokesman for the IFRC. "People are in a weakened condition. They are sick; they are hungry. Without shelter, their whole situation is seriously exacerbated."

Tarpaulin supplies within Myanmar have run out, and the market for items such as shelter materials is very tight because of the demand from China's earthquake victims as well, the U.N. said.

Its ad hoc group to coordinate emergency shelter for Myanmar warned that the "potential for price gouging is high."

The U.N.'s Office for the Coordination of Humanitarian Affairs warned that funding for its logistics operation has to be stepped up. Only $20 million of the $50 million required has been received.

"These funds are needed to extend the outreach of the operation from the hubs to those villages that have yet to receive assistance," said OCHA in its latest situation report.

OCHA also indicated that foreign relief workers still faced hindrances in reaching cyclone victims, especially outside of Yangon.

While U.N. staff face no reported obstacles in obtaining visas to Myanmar, the visa process for international humanitarian groups "still seems to be more cumbersome," and some of them have visa requests that have been pending for up to three weeks.