"We're now in the more tricky period of recovery," said David Nabarro, crisis chief at the World Health Organisation.

"The next three months are key. That's when we have to make sure everything maintains its momentum," he told reporters.

According to official figures, the global death toll from the tsunami was 180,355, although tens of thousands are still listed as missing.

Indonesia's Aceh province was the hardest-hit region, but December's disaster also struck Indian, Sri Lanka, Thailand and other Indian Ocean nations including Maldives and Somalia.

The response to the tsunami was unprecedented, with 12 to 13 billion dollars (9.8 to 10.4 billion euros) in aid pledges to the United Nations, other aid agencies and governments -- although not all donors have made good on their promises.

"There was speed, there was a bold response," said Simon Missiry, head of Asia Pacific department at the International Federation of Red Cross and Red Crescent Societies.

Local Red Cross aid workers were among the first on the scene, bringing relief within half an hour of the disaster -- even though many had lost their families to the tsunami, he added.

Such action, plus the rapid international aid effort, helped ward off epidemics and starvation, which often strike hard in disaster zones, the agencies said.