The head of UNICEF said the tidal waves, sparked by a series of huge undersea earthquakes, had wielded "staggering" power and had left hundreds of thousands of people at risk.

The main UN disaster relief agency said the first teams were sent to Sri Lanka, the nation worst hit by the waves that roared across the Indian Ocean which killed more than 11,000 people in 10 Asian and African countries.

Jan Egeland, the UN Emergency Relief Coordinator, said: "This is the first step in what will surely be a larger United Nations response to catastrophic losses suffered as a result of earthquakes and tidal waves."

The UN Office for the Coordination of Humanitarian Affairs (OCHA) said the disaster countries needed portable sanitation facilities, medical supplies, tents and helicopters to evacuate people.

UN Children's Fund (UNICEF) teams in nations around the Indian Ocean were assessing damage. "The power of this earthquake, and its huge geographical reach, are just staggering," said UNICEF executive director Carol Bellamy.

"Hundreds of thousands of children in coastal communities in six countries may be in serious jeopardy," she said.

A spokesman for UN Secretary General Kofi Annan called the waves "catastrophic" and said Annan had been "profoundly saddened to learn of the massive loss of life and destruction."

Much of the early aid was focused on Sri Lanka which declared a state of emergency and appealed for international help. The Maldive islands in the Indian Ocean had also asked for international help.

A French government plane was to take about 100 doctors, rescue specialists and communications experts to Sri Lanka on Monday. Israel was also sending doctors and has offered similar help to India.

Pakistan said it would send tents, medicine and water to Sri Lanka, where more than 4,300 people were reported dead. More than 2,400 died in India.

The Indian government set up a huge operation to help people on its southern coasts and to take food and medicine to neighboring Sri Lanka.

Five Indian warships headed for Sri Lanka, defence officials told AFP. Three airforce cargo planes loaded with emergency supplies flew to India's stricken Andaman and Nicobar islands in the Bay of Bengal, they said.

The defence ministry said it was deploying naval ships, heavy-lift helicopters and aircraft to speed up rescue operations.

Russia said two transport aircraft would leave Monday for Sri Lanka with a helicopter, tents and other equipment and staff to help victims.

Greece said a military C-130 cargo plane would leave late Sunday for India and Sri Lanka with 11 tonnes of medical supplies and volunteers. Turkey also promised help.

The European Union said it was providing immediate aid of three million euros (four million dollars) to meet "initial vital needs," and that more substantial aid would be provided.

Germany and Ireland each released one million euros for emergency relief and Kuwait offered one million dollars.

Britain sent two experts to join one of the UN crisis assessment teams and pledged 100,000 dollars to fund a World Health Organisation crisis response team.

"For all the huge advances in the control of our lives through science and technology an earthquake on this scale is truly humbling as well as profoundly tragic for everyone involved," British Foreign Secretary Jack Straw said.

The United States also said it was ready to send assistance.

"On behalf of the American people, the president (George W. Bush) expresses his sincere condolences for the terrible loss of life and suffering caused by the earthquake and subsequent tsunamis," the White House said in a statement.

"The United States stands ready to offer all appropriate assistance to those nations most affected including Sri Lanka, the Maldives, Thailand, and Indonesia, as well as the other countries impacted," the statement added.

In Geneva, the International Federation of Red Cross and Red Crescent Societies appealed for 7.5 million Swiss francs (4.8 million euros, 6.6 million dollars) to help an estimated 500,000 survivors.

Caritas Switzerland and the Swiss Red Cross offered a total 400,000 Swiss francs, while Caritas France offered some 100,000 euros.

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