BANGKOK, Thailand — U.N. Secretary-General Ban urged Burma to focus on saving lives, not on politics, after the military government today shunned an American proposal for naval ships to deliver aid to cyclone survivors.

The United Nations says up to 2.5 million survivors of Cyclone Nargis face hunger, homelessness, and potential outbreaks of deadly diseases.

“This is a critical moment for Myanmar,” Mr. Ban told reporters after arriving in Bangkok, Thailand. “The government itself acknowledges that there has never been a disaster on this scale in the history of their country … The issues of assistance and aid in Myanmar should not be politicized. Our focus now is on saving lives.”

Mr. Ban was to fly tomorrow to Burma on a mission to scale up relief efforts and lobby the junta to allow more foreign aid workers into the isolated country. He told reporters his two-day visit would include a trip to areas devastated by the cyclone and talks with officials, including the junta leader, Senior GeneralThan Shwe.

Before leaving U.N. headquarters, Mr. Ban said the world body had finally received permission from the junta to use helicopters to carry aid to stranded victims.

His announcement was not immediately confirmed by officials in Burma.

Burma’s state-controlled media said that American helicopters or naval ships were not welcome to join the relief effort.

The New Light of Myanmar newspaper said accepting military assistance “comes with strings attached” that are “not acceptable to the people of Myanmar.”

The report cited fears of an American invasion aimed at grabbing the country’s oil reserves.

America, as well as France and Great Britain, have naval vessels loaded with humanitarian supplies off the Burmese coast, waiting for approval from the junta. The article did not say whether the French and British supplies would be allowed.

The four American warships in the region were seen as a major potential boost for the relief effort with the capacity to deliver supplies to inaccessible areas of the low-lying Irrawaddy delta, with 14 helicopters, two landing craft vessels, two high-tech amphibious hovercraft, and about 1,000 Marines.

American military aircraft are already sending aid on about five flights a day from Thailand to Rangoon.

A Pentagon spokesman, Bryan Whitman, said today that the four U.S. Navy ships off the coast of Burma will remain there in hopes that officials of the military junta might yet approve delivery of aid by Americanhelicopter.

Asked how long the ships would stay, Mr. Whitman said: “For now, we think the need is compelling enough that we ought to keep those ships there.”

The New Light of Myanmar gave no explanation why the regime was willing to accept aid flown on American C-130 cargo planes, with American military personnel on board, but would not allow the warships and helicopters to deliver relief supplies.

Burma’s xenophobic leaders appear to have long feared an invasion by America, a concern that some analysts believe prompted the junta’s abrupt decision in 2005 to move the capital from Yangon to the remote city of Naypyitaw, which is equipped with bunkers.

Before leaving New York, Mr. Ban welcomed the junta’s “recent flexibility” in saying it will allow relief workers from the 10-country Association of Southeast Asian Nations — of which Burma is a member — to begin distributing aid.

He warned that relief efforts to save cyclone survivors had reached a “critical moment.”

“We have a functioning relief program in place but so far have been able to reach only 25% of Myanmar’s people in need,” he said yesterday.

So far, the few foreign aid workers allowed inside the country have been banned from the areas of the worst devastation in the delta.

At least 78,000 people were killed in the May 2-3 storm and 56,000 remain missing. European Union nations have warned that Burma’s junta could be committing a crime against humanity by blocking aid.

Mr. Ban also planned to attend a meeting of aid donors in Rangoon on Sunday. Burma, one of the world’s poorest nations, claims losses from the disaster exceeded $10 billion.

Burma’s leaders began three days of mourning for the dead and missing yesterday.

The U.N. weather agency said the storm warnings issued by Burma to alert its population about Cyclone Nargis were sufficient and heavy loss of life as the storm hit was inevitable.

Warning times of five to seven days would have been necessary, the director of the U.N. agency’s disaster risk reduction unit, Dieter Schiessl, said. Such early notice is beyond the current state of weather prediction technology, he added.