BANGKOK  Myanmar’s military junta promised on Friday to allow into the country “all aid workers” of any nationality, after three weeks of blocking most assistance to cyclone victims, the United Nations secretary general, Ban Ki-moon, announced.

Mr. Ban’s statement was met with surprise and wariness by relief officials here and American officials in Washington, not least because the pledge was delivered on the eve of an international donors’ conference in Myanmar this weekend.

While the junta has largely ignored international criticism of its response to the May 3 storm, which left some 134,000 people dead or missing, it is asking for $11.7 billion in reconstruction money. That is nearly Myanmar’s annual gross domestic product.

Mr. Ban did not release details of his discussions with the junta or say whether it was now promising a free flow of volunteers and what aid officials say is desperately needed relief.