"Three weeks down the line, I think now we are quite impressed with how rapidly the recovery efforts of people are starting to take place," Margareta Wahlstrom, the UN's special tsunami envoy, told a news conference in the Thai capital.

"We are now reaching very quickly the second phase of our relief operation, which will focus on how to support people to regain their livelihoods -- how to get them back to work, how to make sure the children go back to school, how to prevent the outbreak of diseases," she said.

The shift in focus would lead to a gradual reduction in the military aid effort, especially army helicopters which have been the only way of reaching many isolated tsunami-hit communities, especially in the Indonesian province of Aceh.

Instead, Wahlstrom said aid agencies would be relying more on repaired roads or sea routes to deliver longer-term aid to help hundreds of thousands of survivors get back on their feet.

A simmering 30-year separatist rebellion in Aceh has caused the Indonesian government to fret about the safety of foreign aid workers in the region, although Wahlstrom said the U.N. had no specific worries.

"For the time being we have no security concerns," Wahlstrom said, after three weeks spent assessing the damage in tsunami-hit regions. "Our situation has not changed in Aceh in terms of how we work and how we organise ourselves."

Rebel leaders have repeatedly said they welcome international relief efforts spearheaded by the United Nations and would not attack aid workers or convoys.

The death toll from the killer waves, which thumped into the shores of the Indian Ocean on Dec 26, stands at over 225,000, a figure which is likely to creep up, Wahlstrom said, although a clear picture of the extent of physical damage had emerged.

"I think in Sri Lanka and the Maldives there is a good understanding of the impact," she said.

"And in (Indonesia's) Sumatra it's very quickly getting to the point where there is an understanding of the economic cost, of the social cost, and what needs to be done infrastructurally."

Health officials are still worried about the threat of outbreaks of diseases such as cholera in towns where buildings and infrastructure such as sewage systems have been washed away.

The World Health Organisation (WHO) said it was also keeping a close eye out for increases in infection rates of malaria or dengue fever among communities now living without normal protection from mosquitoes.

"A lot more people might be exposed to mosquito bites because they are displaced," said William Aldis, a Bangkok-based WHO official, who added that a small rise in dengue infection rates in tsunami-hit parts of Thailand was a cause for concern.

"Mosquito breeding may also have increased due to standing water in the damaged areas, so that means we have to be looking for malaria," Aldis said.