By Philippa Fogarty BBC News, Bangkok

Tens of thousands of houses have been damaged or destroyed

In the days after the 2004 Asian tsunami, military rulers in Burma said that they did not need any international help.

Highly suspicious of foreign aid agencies, the regime said that it could provide for the thousands left homeless without any external aid.

But it appears as though the sheer magnitude of the devastation caused by Cyclone Nargis could perhaps force a shift.

With the death toll from the disaster an estimated 15,000, and with hundreds of thousands without shelter, Burma's government looks more willing to accept overseas help.

Shantha Bloemen, a Bangkok-based spokeswoman for Unicef, described the situation in Burma as an "enormous emergency".

"The scale of this disaster is - as we have been discovering in the last 48 hours - just immense," she said.

"It will require a big emergency response with the right equipment and staff - which is a challenge in any situation, but particularly in a situation like this."

"The good thing is that the government has indicated that it welcomes international assistance - that's a good sign.

"The focus now is on getting out there and assessing the scale and nature of the damage."

'Wiped out'

Andrew Kirkwood, Save the Children's country director for Burma, said that responding to the devastation would be a major logistical feat, requiring boats, helicopters and trucks.

"There are seven townships in the southwest of the delta region in which we think 90 to 95% of homes have been wiped out," he said.

HOW YOU CAN HELP Here are some of the major aid agencies working inside Burma The Red Cross Oxfam World Food Programme Unicef Save the Children World Vision

"The main needs that we can estimate right now are for shelter, food, essential drugs, mosquito nets and water purification tablets."

Burmese troops were a visible presence in the main city, Rangoon, where they were trying to get the power back on, he said.

"But local authorities in the areas most affected are probably completely affected like everyone else."

Aid agencies in Burma currently face a difficult operating environment.

Most of their work in the country is done by local staff, because visas for foreign nationals are restricted.

In-country travel is also restricted, and agencies must apply in advance for official permission. In some cases, ministry representatives sit in on agency meetings.

But Mr Kirkwood said that there were signs of flexibility.

Damage can be assessed in Rangoon but some areas remain inaccessible

"It is unclear to us the extent to which normal operating protocols will have to be followed at this time," he said.

"Every indication is that everyone realises that this is an unprecedented event in Myanmar's [Burma's] history and the government is much more open to international assistance than it has ever been."

'Can't wait'

Aid agency World Vision said it had reached an agreement with the Burmese government on visa applications.

"Last night the government said we could send in extra humanitarian help," said an Australia-based spokeswoman for the organisation.

"Normally it takes one month to organise a visa - but obviously with the crisis we can't wait one month - so they have agreed that on a case by case basis they will try to get us visas in two to three days."

She described the move as very positive. But at the moment the agency's priority is working out what needs to be done.

"We know roughly what to expect but every situation is different," she said. "We know we will need food, shelter, we'll have to provide clean water. We certainly won't be calling on people to go in until we have identified exactly what we need."

The motives of the Burmese government in allowing aid are unlikely to be completely altruistic. Turning down international offers of help could have serious repercussions.

The main trigger for last September's anti-government protests was poverty - something that the cyclone will have made worse. Any failure by the authorities to act could potentially lead to further demonstrations.

Moreover, refusing desperately-needed foreign aid would be seen as indefensible in the eyes of the international community - which is, for the most part, already deeply critical of the military regime.

The extent to which the aid agencies will be allowed to operate freely in cyclone-hit regions remains to be seen - especially in certain ethnic-minority areas.

At a news conference in Rangoon, the message was mixed.

"We are greatly thankful to friendly countries that are giving assistance to us,"said Information Minister Kyaw Hsan.

But Social Welfare Minister Maung Maung Swe said foreign aid teams would still need to negotiate with ministry officials for access, the French news agency AFP reported.

Most organisations are in the early stages of planning their response.

And only the coming days will reveal the extent of their task - and whether they will be allowed to do it.