Myanmar's military government Saturday came under renewed international pressure from rights groups and the US defence chief who said its slow response to the cyclone disaster had cost "tens of thousands of lives."

US Defence Secretary Robert Gates criticised the military government's delay in allowing in foreign aid, saying US ships and aircraft could have swiftly brought much-needed relief to the cyclone-hit nation.

"Our ships and aircraft awaited country approval so they could act promptly to save thousands of lives -- approval of the kind granted by Indonesia immediately after the 2004 tsunami and by Bangladesh after a fierce cyclone just last November," Gates told a top-level security forum in Singapore.

"With Burma (Myanmar), the situation has been very different -- at a cost of tens of thousands of lives."

Rights groups also accused Myanmar authorities of forcing victims out of emergency shelters and back to their devastated villages -- even if they have no home left in the wake of the May 2-3 storm.

With tens of thousands of people now living in schools, Buddhist monasteries and tented camps, advocacy groups said they had received reports the government was forcing people to leave the shelters.

The United Nations says so far it has not been able to verify whether people are being forced out, but the charges added to the international frustrations at the slow progress in delivering aid to 2.4 million victims.

Cyclone Nargis left 133,000 people dead or missing when it pounded into Myanmar four weeks ago, destroying entire villages and laying waste to the country's most important rice-growing region.

Nearly one month after the storm, only 40 percent of people in need have actually received any help, according to the United Nations.

UN Secretary General Ban Ki-moon visited here one week ago, announcing that Myanmar's military rulers had agreed to allow foreign aid workers into the hardest-hit regions.

Since then, relief workers have made small successes in delivering assistance.

The United Nations says all the visas for foreign workers it has requested have been granted, and the head of the World Food Programme, Josette Sheeran, visited Myanmar on Saturday to assess the relief operation.

The government announced in state media Saturday that a new coordinating body -- comprising officials from Myanmar, the United Nations, and neighbouring countries in Southeast Asia -- had officially begun working.

But Human Rights Watch and Refugees International said they had received alarming reports of people being forced out of government-run emergency camps and left to fend for themselves amid the storm's rubble.

"It's unconscionable for Burma's generals to force cyclone victims back to their devastated homes," said Brad Adams, Asia director at Human Rights Watch.

"Without shelter, food, and clean water, the government's suggestion amounts to sending people to their deaths and is courting a greater disaster," he said.

The regional head of the UN's disaster response arm, Terje Skavdal, said that he could not confirm those reports but that the UN opposed any forced resettlements.

"It is not only a question of putting people back into their villages, it is also a question of being able to service them with relief," he said.

The military has ruled this country, formerly known as Burma, since 1962. The reclusive generals are deeply suspicious of the outside world, and have only reluctantly allowed in foreign aid workers.

The military rulers have flatly refused to accept American, British and French naval ships laden with relief supplies.

French Defence Minister Herve Morin told AFP in an interview in Bangkok that the refusal of the French ship Mistral "was the reaction of a regime that by its nature fears foreign influence. We did what we could."

"A country that experiences this kind of drama and refuses help from countries which first and foremost want to join in humanitarian relief -- that's a surprising thing, to put it mildly," Morin said.

Singapore's Prime Minister Lee Hsien Loong said Myanmar's response to the cyclone was "regrettable," saying a speedier relief effort could have minimised the suffering of the victims.