Rescuers carry the dead body of a child victim of the devastating floods in Cagayan de Oro City, in southern island of Midanao

Rescuers struggled to help survivors and a ravaged city prepared for a mass burial as the death toll from devastating flash floods in the southern Philippines rose past 650 on Sunday, reports AFP.

With hundreds more still listed as missing, tropical storm Washi left Philippine territory after dumping heavy rains that overwhelmed rivers in the port cities of Cagayan de Oro and Iligan on Mindanao island at the weekend.

Iligan, where more than 200 people were killed, was preparing to bury unclaimed bodies in a mass grave as early as Monday because of their advanced state of decomposition.

City health officer, Liddy Villarin said body bags will be marked for possible exhumation.

"We will put markings on the cadaver bags which will give the physical features of each body before they put them in the mass grave," she said.

Entire villages were swept away by floodwaters as residents, normally spared from typhoons that ravage other parts of the Philippines every year, slept in the early hours of Saturday despite storm warnings.

The Philippine Red Cross said 652 people had been confirmed dead by its field staff and another 808 were currently listed as missing.

Gwendolyn Pang, the organisation's secretary-general, told AFP that bodies buried in mass graves will have to be properly marked, photographed and mapped for future identification.

The head of the government's disaster response agency, Benito Ramos, earlier said its own count stood at 516 deaths and 274 missing but conceded that the toll was likely to rise.

"I'm out here retrieving bodies that are starting to rise to the surface," Ramos told AFP by mobile phone from a rescue boat off Cagayan de Oro.

President Benigno Aquino is set to visit the disaster zone on Tuesday after ordering a review of the country's disaster defences.

Pope Benedict XVI prayed for the victims of the latest natural disaster to hit the largely Roman Catholic archipelago, which is also prone to earthquakes and volcanic eruptions.

The United States offered assistance as Manila appealed for help to feed, clothe and house more than 35,000 people in evacuation centres.

Secretary of State Hillary Clinton said in a statement: "The US government stands ready to assist Philippine authorities as they respond to this tragedy."

China was one of the first countries to announce cash donations.

A 20,000-strong military force normally involved in fighting Muslim insurgents in Mindanao was leading rescue and relief operations but the efforts were hampered by masses of mud left behind by floodwaters.

An AFP photographer saw a 30-member military and police rescue team landing Sunday in Bayug, a delta area near Iligan that was formerly home to a fishing community estimated by social welfare officials to have had up to 1,000 residents.

The delta had been swept clean of most structures, leaving those left alive having to rebuild huts with scrap wood.