YANGON, Myanmar - U.N. helicopters loaded with relief supplies reached areas of Myanmar's Irrawaddy Delta yesterday that have been cut off from regular deliveries of aid since a devastating cyclone five weeks ago, an official said.

Four of the five aircraft that arrived over the weekend got to work shuttling rice, water-purification systems, and other emergency supplies to villages around the hardest-hit towns of Bogale and Labutta, said Paul Risley, a spokesman for the U.N. World Food Program.

U.N. officials and aid groups have criticized Myanmar's military regime for restricting access to the delta, saying it has prevented enough food, water and shelter from reaching survivors.

Until now, the United Nations had only one helicopter operating in Myanmar. It made six trips last week, Risley said. Supplies were mainly being delivered by boats that took several hours to navigate short distances in the delta's network of waterways.

"Today was the first day where you really saw a multiplier effect," Risley said, adding that the helicopters reached four villages in the morning. "These are areas that clearly have not received regular supplies of food or other relief assistance."

Four more helicopters chartered by the World Food Program, which are currently in neighboring Bangkok, Thailand, are expected to fly to Myanmar this week. That will bring the U.N. agency's total number of helicopters in the country to 10, he said.

The relief effort still faces myriad problems, including a severe shortage of housing materials that could leave hundreds of thousands of survivors exposed to heavy rains as the monsoon season begins, the WFP and other aid agencies say.

"There's clearly a need for tarps and other roofing material, for anything that can help them rebuild their houses," Risley said, noting that rains have left many delta villages knee-deep in mud.

The United Nations estimates a total of 2.4 million people were affected by Cyclone Nargis and warns that more than one million of those still need help, mostly in hard-to-reach spots in the Irrawaddy Delta.

The cyclone killed more than 78,000 people in the impoverished country.

In Geneva, Switzerland, yesterday, the United Nations' expert on human rights in Myanmar said he was worried about the arrest Wednesday of a well-known comedian who was trying to help cyclone survivors.

Tomas Ojea Quintana, the U.N. Human Rights Council's new investigator for Myanmar, said he had asked the government for clarification about the arrest of comedian Maung Thura, whose stage name is Zarganar.

The 46-year-old comedian and his team had made videos of their relief activities, and Zarganar gave interviews critical of the government's relief effort to foreign media, including the BBC, whose news broadcasts are popular in Myanmar.