YANGON, Myanmar (AP) — Hundreds of experts began assessing the needs of Myanmar's cyclone victims Tuesday 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 United Nations office in the commercial capital of Yangon to plead for help.

Some 250 experts from the U.N., 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 Bangkok, Thailand.

The United Nations estimates more than 1 million of the storm's survivors, mostly in the delta, still need help. Cyclone Nargis 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.

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

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 Tuesday 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 group from Dagon township on the outskirts of Yangon was bundled into a waiting police car and remained in detention, witnesses said.

Pitt said she was unaware of the arrests.

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 Nobel Peace Prize laureate and pro-democracy leader Aung San Suu Kyi.

On Tuesday, the junta released 15 members of Suu Kyi's party who were detained last month for demanding her release, a party spokesman said.

The 15 were shoved into police trucks and hauled away May 27 after they marched from the National League for Democracy's headquarters to her house to demand her freedom. They were held at a compound that has been used to hold hundreds of Buddhist monks and other citizens following pro-democracy protests in September that were violently suppressed by the military.

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

The European Union's envoy to Myanmar, Piero Fassino, said Tuesday that the focus on humanitarian assistance for cyclone victims "cannot sideline the importance of political problems that are still there."

Repeated attacks on the junta's rights record has fueled an intense xenophobia among the generals. That suspicion of foreigners has 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.

The country's top leader, Senior Gen. Than Shwe, promised U.N. Secretary Secretary-General Ban Ki-moon last month that he would improve access but until now only a trickle of foreign aid workers and relief supplies had reached survivors.

Pitt, the U.N. spokeswoman, said she was hopeful the start of the assessment signaled a greater willingness on the part of the government to work with aid workers to reach survivors and allow a better understanding of their needs.

The assessment began a day after U.N. helicopters loaded with relief supplies started reaching areas of the delta that were cut off from regular aid since Cyclone Nargis struck.

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

A total of four flights flew Monday to seven locations in the delta and six more sites were expected to be reached Tuesday, he said.

Until this week, the U.N. had only one helicopter operating in Myanmar, Risley said. Most supplies were being delivered by boats that took hours to travel short distances in the delta's network of waterways.