The UN Secretary-General, Ban Ki-moon, on Thursday tried to call Snr-Gen Than Shwe to urge him to postpone the scheduled May 10 referendum on the draft constitution and to focus on the relief effort aimed at the survivors of Cyclone Nargis.

Ban had written to the reclusive general two days earlier with the same message. So far, he has received no reply to both of his efforts at communication.

UN aid officials continue to express frustration over their lack of ability to get visas to allow assistance teams to enter Burma.

Ban, who was enroute to Georgia, told reporters that he called Snr-Gen Than Shew but was not put through to him directly.

"I expect that I will be able to speak to him, but nothing has been scheduled yet," Ban  said.

Meanwhile, top UN officials said they have been unable to make much progress in relief and aid operations, because of the bureaucratic hurdles posed by the military junta.

The secretive, xenophobic country has been dragging its feet over allowing relief teams to enter the country and pressing its demands that money and relief be delivered directly to the military government, officials said. The military government has a long history of fear of outsiders.

"I am deeply concerned by the inability of many food aid and international aid workers not being able to deliver as soon or as much as we had hoped," Ban told reporters in Georgia.

"I am urging again at this time the Myanmar authorities to do that, to first of all issue visas expeditiously, and if possible, exempt all visa requirements for all UN aid workers, so that we can reach out to needy people," he said. More than 40 visa applications are pending with the Burmese authorities right now, he said.

"I am concerned that if we lose time, then many more people will die because of this crisis. I am also concerned that disease may spread. This we must prevent as soon as possible."

Earlier in the day, John Holmes, under-secretary-general for Humanitarian Affairs and United Nations Emergency Relief, also expressed his frustration that relief material had not reached Burma.

"Frustration is growing that the humanitarian response is being held back because of difficulties of access in different ways,” he said. “There has been limited progress since yesterday, but not nearly as much as was needed. Conditions on the ground are desperate.”

Holmes said two members of the Disaster Assessment and Coordination Team were now in Yangon, but two others had not been allowed in for reasons yet to be established, but which might have to do with their passports.

"World Food Programme (WFP) staff has received two new visas, but the picture is still very patchy and extremely unsatisfying," he said.

Holmes said: "Since yesterday, the authorities had agreed that customs charges and clearances should be waived for aid delivery, but it was not clear whether that had been made fully operational on the ground."

There had been "a little bit of progress" in terms of opening roads and establishing piped water and electricity to some areas of Rangoon and more supplies continued to arrive from Thailand, Japan, China and India, he said.

Holmes said: "The border is not closed, but it simply is not anything as open as it should be."