Thai oceanographer Sakanan Plathong said a preliminary survey showed 20 percent of the reefs examined around the Similan islands, an archipelago with world-class dive sites, were destroyed.

But more than half of the reefs had been knocked over during the tsunami, which churned up sand on the ocean floor, removing reefs' foundations and causing them to fall.

If divers managed to move fallen reefs back into an upright position within a month, the coral should survive and keep growing, Sakanan said.

"Based on the initial survey, we estimate that up to 20 percent of reefs (in the Similans) were destroyed, but some 60 percent of reefs fell over and need to be turned upright," he said.

On Tuesday the United Nations Development Program (UNDP) said a disaster assessment mission found that on average just five percent of the coral reefs along the coast and around the main tsunami affected islands had been damaged.

UNDP said the reefs were littered with potentially deadly debris and needed to be cleaned, and said reefs around the Similan and Phi Phi islands were particularly hard-hit.

It also determined that future development of sustainable eco-tourism and the recovery of fishing communities will rely heavily on the restoration and protection of coral reefs.

Sakanan said he wants to recruit a first group of 40 volunteer divers to lift fallen reefs into healthy positions this week, and hopes eventually to find another 150 volunteers.

If the fallen reefs are not moved, up to 40 percent of the reefs could be destroyed, said Sakanan, a professor at the Prince of Songkhla University.

The survey conducted last week was sponsored by the Tourism Authority of Thailand, the National Park Department, and dive associations in Phuket and Bangkok.

Sakanan also urged the Natural Resources and Environment Ministry to at least temporarily ban diving at three sites including the Chinese Wall, Christmas Point and Snapper Alley, all around the Similan islands.

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