Sri Lankan air force helicopters which have been airlifting some 24 tonnes of food daily to ravaged areas in eastern Ampara district were grounded by rains Tuesday, police said.

"Only one helicopter could take off today," said police inspector Upul Jayawardena, heading the operational centre in Ampara district, where some 8,000 people were swallowed by the waves.

But two US Blackhawk helicopters were seen flying near the international airport outside Colombo as dozens more US marines arrived on the island to boost a deployment that will eventually total 1,500 troops, an official said.

US embassy spokesman Chris Long confirmed US helicopters "are already on the ground and more troops have arrived", adding they were being deployed "purely for humanitarian work."

The Sri Lankan air force had been carrying out about two dozen flights a day from Ampara since the tsunami battered the island on December 26. The latest government figures raised the death toll to 30,229 with 3,858 still missing.

Some 861,016 have been displaced.

Jayawardena added that trucks carrying supplies were unable to reach some areas due to the rain, bringing relief work to a standstill.

Besides adding to the misery of the hundreds of thousands left homeless in Ampara, government spokeswoman Tara de Mel warned the rains and flooding had raised fears of waterborne disease.

She said urgent efforts were under way to get safe drinking water to those sheltering in makeshift camps and schoolrooms.

A Japanese volunteer doctor told AFP the rains were preventing many of the ill from reaching emergency medical centres.

"On a day like this, when it is raining steadily, it becomes difficult to attend to the patients," said Shoichi Nakano, who heads a team of 20 Japanese medical volunteers in the mainly-Muslim eastern coastal town of Kalmunai, in the heart of Sri Lanka's rice bowl.

His colleague, Yamashita Tomoko, warned that survivors were increasingly showing signs of stress as the harsh reality of their losses begins to hit home.

"In the last few days (survivors) have been going back to their houses and on seeing the destruction they are completely broken," Tomoko told AFP.

"They have lost everything that they had built all these years -- their house, family members ... everything. These hard realities are now hitting them."

The devastation and massive loss of life was causing psychological depression as well as ailments such as diabetes and hypertension, Tomoko said, adding that her team lacked the medicine to treat the stress-related disorders.

Health ministry officials said isolated cases of diarrhoea had been reported from various camps in the stricken areas but that there were no signs of an epidemic.

Two people at a welfare centre in the south of the country were diagnosed with chicken pox but they were quickly isolated, according to the Daily Mirror newspaper, quoting health sources.

The World Food Programme (WFP) said that despite the bad weather, it was now feeding 290,000 people left homeless by the tsunami.

"We've increased daily truck dispatches from 10 to 12 in the first few days after the disaster to more than 30," said Jeff Taft-Dick, WFP country director in Sri Lanka.

"The delivery challenges are still enormous -- from flooded roads to washed out bridges to continued rains -- but we are gaining greater access every day."

Relief operations are also in full swing in northern and eastern areas controlled by Tamil Tiger rebels, where schools have been converted into camps and medical teams have been tending to the health and sanitation needs of the displaced, correspondents said.

UN Children's Fund chief Carol Bellamy said in Colombo Monday while most afflicted areas had been reached, "the hardest work is still ahead" as relief workers were still trying to access some remote areas where the suffering among survivors "is severe."

Also due in Sri Lanka in the coming days is UN Secretary General Kofi Annan and US Secretary of State Colin Powell, officials said.

Meanwhile, a new threat emerged in the form of landmines planted during three decades of conflict between government troops and Tamil Tiger rebels which have been shifted by the tsunamis, the Relief and Rehabilitation Ministry said.

Five mines were found outside an army camp in Kallady and defused, it said in a statement.

Press reports said mines had also shifted in eastern Batticaloa, and northern Jaffna and Mullaitivu districts, controlled by the rebels but all had been defused.

The ministry contended that the threat posed by the mines was not much more than what existed before the tsunamis struck.

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