YANGON, Myanmar - Hundreds of experts began assessing the needs of Myanmar's cyclone victims yesterday as the country's military junta finally gave them access five weeks after the disaster.

But that improved access was undermined by reports the isolationist government had arrested 18 survivors who were on their way to the U.N. office in Yangon to plead for help.

About 250 experts from the United Nations, the Myanmar government, and Southeast Asian nations headed into the Irrawaddy Delta on trucks, boat and helicopters for a village-by-village survey, the United Nations said.

Over 10 days, they will determine how much food, clean water and temporary shelter the 2.4 million survivors need along with the cost of rebuilding houses and schools and reviving the farm-based economy.

"It has taken quite a long time, but this shows the government is on board by its commitment to facilitate the relief operation and the scaling up that people are asking for," said Amanda Pitt, a U.N. spokeswoman in Thailand.

The United Nations estimates more than one million of the storm's survivors, mostly in the delta, still need help. Cyclone Nargis, which struck May 2, killed more than 78,000 people in impoverished Myanmar.

The information collected will be released in a report next month by the Association of Southeast Asian Nations and should motivate more countries to donate to the cyclone relief operation, Pitt said.

"Many donors said they were ready and willing to provide funding for relief operations and logistics," she said, "but they wanted more access and more comprehensive assessments."

The ruling junta has been sharply criticized by foreign governments and aid agencies for its ineptness in handling the disaster. It also has come under particular fire for forcing survivors from camps and allegedly dumping them in their destroyed villages.

Authorities detained 18 women and children yesterday as they walked to U.N. offices to complain about not receiving any government assistance, according to a government official who refused to be identified for fear of retaliation by the leadership.

The criticism of the junta's aid effort comes on top of long-standing concerns about its poor record on human rights, including its detention of Aung San Suu Kyi, the Nobel Peace Prize laureate and pro-democracy leader. Yesterday, the junta released 15 members of Suu Kyi's party who were detained last month for demanding her release, a party spokesman said.

Suu Kyi has been held in house arrest for more than 12 of the last 18 years. Last month, the government extended her detention by another year.

Repeated attacks on the junta's rights record have fueled an intense xenophobia among the generals. That suspicion of foreigners contributed to the junta barring most international aid groups from the delta until now and rebuffing offers from the U.S. military to help in the relief effort.