The United Nations believes that at least 1.5 million people in Burma have been "severely affected" by Cyclone Nargis, U.N. humanitarian affairs chief John Holmes said on Thursday.

Speaking to reporters at U.N. headquarters, Holmes said he was disappointed at the slow progress made in getting U.N. relief workers and aid shipments into Burma since Wednesday and described cooperation with the country's government as "patchy" and "still extremely unsatisfactory."

U.N. Secretary-General Ban Ki-moon was planning to talk directly with the military junta's senior general, Than Shwe, to persuade him to remove obstacles, Holmes added.

Ban intends to "to urge him very strongly to facilitate access and to allow us to do the job we need to do," he said.

Holmes said two members of an initial disaster assessment team had been allowed into the country but two more who were trying to enter with U.N. documents had been turned back when they arrived at the airport. All four were from ASEAN countries that have reciprocal visa agreements with Burma.

"That's not an acceptable situation," Holmes said, adding that U.N. "laissez-passer" documents should be enough to ensure access, particularly in a crisis situation.

"That's extremely disappointing," he said, reiterating a call for a waiver of visa restrictions to allow international experts in disaster relief into Burma.

State Myanmar radio and television, the main official sources for casualties, reported a death toll of 22,980 with 42,119 missing and 1,383 injured in Asia's most devastating cyclone since a 1991 storm in Bangladesh that killed 143,000.

A U.S. diplomat in Burma said diplomats there were receiving information that there may be over 100,000 deaths.

It was unclear why the government would be worried about U.N. aid workers.

"The authorities have not given an explanation of what their concerns are," Holmes said, adding that the junta was and "isolated and suspicious regime".