The death toll from storm-triggered flash floods that devastated a wide swath of the country's south has risen to 652, the Philippine Red Cross says.

The death toll would most likely rise, Philippine Red Cross Secretary General Gwendolyn Pang said Sunday, with 808 people still missing and many villages remaining isolated and unreached by overwhelmed disaster-response personnel.

Pang says the hardest-hit cities were Cagayan de Oro city, where at least 239 people died, and nearby Iligan, where Red Cross aid workers reported 195 dead, mostly children and women.

Tropical Storm Washi started to blow away toward the South China Sea on Sunday, allowing the weather to clear and disaster-response contingents to intensify search-and-rescue work.

The floods struck while the country slept, forcing survivors to their rooftops and turning two coastal cities into muddy, debris-filled waterways that were strewn with overturned vehicles and toppled trees.

Defence Secretary Voltaire Gazmin and top military officials were to fly to the worst-hit city of Cagayan de Oro on Sunday to help oversee search-and-rescue efforts and deal with thousands of displaced villagers, as the weather began to clear and floodwaters receded. Among the items urgently needed are coffins and body bags, said Benito Ramos, who heads the government's disaster-response agency.

"It's overwhelming. We didn't expect these many dead," Ramos said.

A resident rummages through debris following a flash flood that inundated Cagayan de Oro city, Philippines on Dec. 17, 2011. A tropical storm triggered flash floods in the southern part of the country, killing scores of people. (Froilan Gallardo/Associated Press)

Army officers reported unidentified bodies piled up in morgues in Cagayan de Oro city, where electricity was restored in some areas, although the city of more than 500,000 people remained without tap water.

Most of the victims were asleep Friday night when raging floodwaters cascaded from the mountains after 12 hours of rain from a late-season tropical storm in the southern Mindanao region. The region is unaccustomed to the typhoons that are common elsewhere in the archipelago.

Ayi Hernandez, a former congressman, said he and his family were resting in their home in Cagayan de Oro late Friday when they heard a loud "swooshing sound" and water quickly rose ankle-deep inside. He decided to evacuate to a neighbour's two-storey house.

"It was a good thing, because in less than an hour the water rose to about 11 feet," filling his home up to the ceiling, he said.