DHAKA (Reuters) - A severe cyclone has killed more than 580 people in Bangladesh and left thousands injured or missing, triggering an international relief effort on Friday to help the disaster-prone country cope with its latest emergency.

Local officials and Red Crescent workers said 587 deaths had been confirmed. Hundreds more were injured or missing after Cyclone Sidr struck overnight packing winds of 250 kph (155 mph).

The Category 4 cyclone triggered a 15-foot (5-metre) high tidal surge that devastated three coastal towns and forced 3.2 million people to evacuate, officials and aid agencies said.

“The death count is rising fast as we get more information from the affected districts,” a food and disaster ministry official said. He put the official death toll at 250.

The death toll might rise sharply when search operations end in a half dozen tiny offshore islands inhabited by thousands of fishermen, and coastal areas where telecommunications were cut.

Navy rescuers aboard five vessels had reached the islands and started giving relief to survivors and retrieving bodies, the defense ministry said.

“It will take several days to complete the search and know the actual casualty figure and extent of damage to property,” said disaster ministry official Ayub Miah.

Dhaka and major cities across the country were plunged into darkness at sunset. Many power plants and most distribution systems were damaged by the cyclone and it could take three more days to restore services, power department officials said.

“I am concerned about security. Thugs will have a great time throughout the night,” said businessman Abul Hashem.

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The tidal surge inundated Patuakhali, Barguna and Jhalakathi, cutting off links to the three towns. An official in Dhaka had no information yet about casualties from the area.

The U.N. Office for the Coordination of Humanitarian Affairs (OCHA), told reporters in Geneva 1,000 fishermen were missing.

“Significant damage is expected. However, information collection on casualty and damage figures is still very much in the early stages,” OCHA spokeswoman Elisabeth Byrs said.

MISSING FISHING BOATS

In New York, OCHA chief John Holmes said his office was ready to make available “several million dollars” in emergency aid, but he declined to name a specific figure.

At least 150 trawlers have been reported missing. Though authorities had broadcast repeated storm warnings, many of the missing boats might have been small vessels without radios.

The navy launched search and rescue operations, while four helicopters loaded with emergency relief supplies have been dispatched to some of the worst-hit areas, officials said.

Around 30,000 volunteers mobilized by Bangladesh Red Crescent beat drums and used bullhorns or a special flag system to spread evacuation warnings, the International Federation of Red Cross and Red Crescent Societies said in Geneva.

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The U.N.’s World Food Programme said it was sending 98 metric tons of high-energy biscuits, enough for 400,000 people for three days.

“The urgent needs are food, water purification tablets and medicines,” WFP spokeswoman Christiane Berthiaume 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 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 elections before the end of next year.

By early Friday the storm had weakened to a tropical storm.

Agriculture officials said rice and other crops in the cyclone-battered areas had been badly damaged.

Storms batter the country every year. A severe cyclone killed half a million people in 1970, while another in 1991 killed 143,000. Many of the country’s 140 million people live around low-lying river deltas vulnerable to tidal surges.