A severe cyclone has killed more than 500 people in Bangladesh and left thousands injured or missing, triggering an international relief effort to help the army-backed interim government to cope with the disaster.

Local officials and Red Crescent workers said on Friday 508 deaths had already been confirmed while hundreds more were injured or missing after the cyclone struck overnight packing winds of 250 kph (155 mph).

The Category 4 cyclone also triggered a 15-foot (5-metre) high tidal surge that devastated three coastal towns and forced

evacuation of hundreds of thousands of people, officials and aid agencies said.

The death count is rising fast as we get more information from the affected districts, an official at the food and disaster ministry said.

But he put the latest official confirmed death toll at 247. It may go much higher, he said, adding that some areas had yet to be reached.

It is impossible to describe how serious was the devastation wrought by the killer cyclone, a television reporter said.

1,000 fishermen remain missing as of last night in Bangladesh, Elisabeth Byrs, spokeswoman of the U.N. Office for the Coordination of Humanitarian Affairs (OCHA), told a news briefing in Geneva.

Some 150 trawlers had been reported missing.

Significant damage is expected. However, information collection on casualty and damage figures is still very much in the early stages, she said.

Fakhruddin Ahmed, chief of the army-backed interim government, flew to devastated coastal districts on the Bay of Bengal on Friday to see the extent of the damage, officials said.

The cyclone, which followed devastating floods in July-September that killed more than 1,000, posed a new challenge to the interim administration, whose main task is to hold free and fair national elections before the end of next year.

The cyclone triggered a tidal surge that inundated the towns of Patuakhali, Barguna and Jhalakathi, cutting off communication links. A government official in Dhaka said there was no immediate information about casualties from the area.

Nearly 1 million people were evacuated in 13 coastal districts, officials said.

Bangladeshi authorities dispatched four helicopters loaded with emergency relief supplies including dry foods, medicines and blankets to some of the worst-hit areas, officials said.

The United Nations World Food Programme, in its initial assistance, has begun distributing 98 tonnes of high-energy biscuits to storm victims.

NOW A TROPICAL STORM

By early Friday the storm had weakened to a tropical storm and had moved well inland northeast of Dhaka.

The Bangladeshi navy has launched a search and rescue operation while helicopters began ferrying relief supplies to offshore islands, the defense ministry said.

Most deaths were caused by collapsing houses and flying debris. Three people were electrocuted in the capital.

Agriculture officials said rice and other crops in the cyclone-battered areas had been badly damaged, causing added suffering to villagers who had earlier lost two crops in the floods.

Life shall never be easy, said Mohammad Salam, a farmer in Khulna. We are destined to suffer.

Storms batter the poor south Asian country every year. A severe cyclone killed more than half a million people in 1970, while another in 1991 killed 143,000.

Hundreds of fishing boats caught in the cyclone failed to return to shore. Across the devastated regions, trees and power poles were uprooted, disrupting communication and electricity supplies.

We have been virtually blacked out all over the country, said a disaster management official in southern Mongla, another of the worst affected areas.

Television news reports said more than 100 fishing boats in the Bay of Bengal had failed to return to shore. Though authorities had broadcast repeated storm warnings, many of the missing boats might have been small vessels without radios.

The cyclone blew past India's eastern coast without causing much damage, police and weather officials there said on Friday.

(Additional reporting by Nizam Ahmed and Azad Majumder in Dhaka and Reuters stringers in Barisal and Khulna, Bappa Majumdar in Kolkata and Stephanie Nebehay in Geneva; editing by Roger Crabb)