Forecasters sounded alarms Wednesday over a new storm headed for the Philippines, even as workers repaired seawalls demolished by a typhoon that killed at least 21 people and left scores stranded in swamped communities.

Typhoon Nesat also left 35 people missing and brought some of downtown Manila's worst flooding in decades before blowing out of the northern Philippines early Wednesday toward southern China with winds of 120 kilometres an hour.

Floodwaters were receding in most places, but many low-lying communities in the north remained in crisis. Weather forecasters, meanwhile, said another storm was gathering force and could hit soon.

Manila

Mayor Santiago Austria of the rice-farming town of Jaen in Nueva Ecija province appealed to the government for help, saying many people in his community of 63,000 needed to be rescued but that officials there had only four rescue boats.

"Many people here are still on top of their houses. We don't have enough boats to reach them and hand them food," Austria said.

Civil Defence Office chief Benito Ramos said army troops were on their way to assist Jaen.

The town of Obando, north of Manila, remained under waist-high water and officials had not yet been able to check on reports of houses swept away on two nearby islands where thousands of residents live, Mayor Orencio Gabriel said.

In all, 320,000 people were affected by the storm, with 73,000 in evacuation centres and about 100 still stranded, officials said.

Meanwhile, a fresh tropical storm was brewing in the Pacific, government forecaster Bobby Javier said, adding that it already had sustained winds of 85 km/h and gusts up to 100 km/h and was expected to strengthen significantly before hitting major parts of the country in the next few days.

Ramos said disaster agencies were being kept on full alert because of the new storm.

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Workers remove fallen trees along Manila's bayside which was flooded due to strong currents from Typhoon Nesat as it hits the Philippines on Tuesday. ((Aaron Favila/Associated Press))

In Manila, hundreds of workers used cranes to lay sandbags where parts of a downtown seawall were ripped off by the typhoon's huge waves and fierce winds. Residents made repairs to nearly 5,000 houses damaged in the storm.

Emergency services and residents in the Philippine capital cleaned up and restored electricity Wednesday after a powerful typhoon unleashed floodwaters and fierce wind that killed at least 20 people and sent huge waves crashing over seawalls.

Most deaths from the typhoon occurred in and around metropolitan Manila, which already was soaked by heavy monsoon rains ahead of Tuesday's arrival of Nesat.

Nesat unleashed torrents of floodwaters that swamped Manila's downtown areas, rapidly engulfing hotels, a hospital, the U.S. Embassy, business offices and several blocks of residential areas in waist-deep floodwaters.

On Wednesday, scavengers rummaged through household items carried by the floodwaters footwear, a basketball, a child's school bag, a hunter's hat. Mar Depas, 28, said he collected about a dozen fairly new leather shoes and sandals but was disappointed that they didn't match.

"I can't find their pairs. They're useless," Depas said. "I came late ... most of the better recyclable stuff is gone."

Power was gradually restored Wednesday to Manila's downtown area, which strewn with trash and fallen bamboo pieces washed ashore by storm surges. City trains resumed operations.