The junta in the former Burma has allowed relief flights to deliver supplies to Yangon, the largest city, but had balked at aerial access to the south-western Irrawaddy Delta, where an estimated 2.4 million people were left destitute.

"We have received government permission to operate nine WFP helicopters which will allow us to reach areas that have so far been largely inaccessible," UN Secretary-General Ban Ki-Moon told reporters before departing for a visit to Myanmar.

Amid warnings that many more people could die in the Southeast Asian country, Ban said he welcomed the government's "recent flexibility" but added that aid workers had so far been able to reach only around 25 percent of those in need.

Ban said a donors' pledging conference in Yangon on Sunday would be crucial for longer term rebuilding.

The official toll is 77,738 people killed and 55,917 missing, one of the worst cyclones to hit Asia in decades. Myanmar's government has estimated the damage at $10 billion.

Ban, due to arrive in Thailand on Wednesday and go to Myanmar on Thursday, said he hoped junta supremo Than Shwe would be among senior government officials he meets.

WAKE-UP CALL

The declaration of a mourning period, after the first post-cyclone visit to the delta by 75-year-old Than Shwe on Monday, was taken as a possible sign the leadership had awoken up to the scale of the catastrophe.

"The old man must have been shocked to see the real situation with his own eyes," one retired government official said in Yangon, where the start of the monsoon season has caused more flooding and misery for storm survivors.

Than Shwe, who has run Myanmar since 2005 from Naypyidaw, a new capital 250 miles (390 km) north of Yangon, was shown on state TV touring hard-hit towns on Monday and again on Tuesday, offering words of encouragement and giving orders.

UN humanitarian chief John Holmes, visiting Myanmar, said military-run camps in the devastated Irrawaddy Delta "seemed well organized" but that most survivors had no shelter.

"There are still a lot of supplies needed to get in in the future in terms of food, but not just for now but for some months to come," Holmes told reporters after meeting Prime Minister Thein Sein.

Scot Marciel, the US envoy to the Association of Southeast Asian Nations, of which Myanmar is a member, said the junta's response to the disaster has been "appalling" with hundreds of thousands of lives at risk.

"The door must be opened far wider -- and rapidly -- to prevent a second catastrophe," he told a congressional hearing in Washington, adding that responsibility "will fall squarely on the shoulders" of Than Shwe and Myanmar's other leaders.

In Tokyo, Myanmar's ambassador told the foreign ministry that Japanese relief workers would be allowed in, a ministry official said.

SUSPICIONS

Until the last few days, the junta's attention appeared to have been on a May 10 referendum on a constitution drafted by the army intended to precede multiparty elections in 2010. The vote was postponed to May 24 in areas worst-hit by the storm.

The military, which has ruled the country in various incarnations for 46 years, historically has been suspicious of foreign interference. That distrust has deepened since the wave of international outrage and tighter sanctions following last year's crackdown on pro-democracy demonstrators.

The New Light of Myanmar, the junta's main mouthpiece, quoted Than Shwe as saying the government "took prompt action to carry out the relief and rehabilitation work."