Tropical depression 'Merbok' caused severe flooding and landslides after hitting the main island of Luzon on Tuesday, just days after tropical storm 'Muifa' left a wider trail of destruction and sunk boats before roaring on to southern Vietnam.

The latest of the two storms, which slammed into the east coast of Luzon and dumped heavy rain, has left 18 people dead, 14 injured and six missing.

Muifa cut a swathe of destruction across the country at the weekend, claiming 61 lives, injuring 101 people and leaving 80 missing.

The storms smashed nearly 34,000 houses and displaced more than 314,000 people, the weather bureau here said.

Authorities said Wednesday they were concerned about the fate of the missing people, most of whom were fishermen or tugboat operators from nearly 50 boats that were capsized or sunk by Muifa.

They include 42 fishing boats off Mindoro island and two tugboats off Romblon island in the central Philippines, four small outriggers off the town of San Francisco southeast of Manila, and one boat off the northwestern town of Infanta.

The coastguard said it rescued 11 and retrieved one dead crewman from the tugboats, but there have been no news from those aboard the smaller vessels.

The civil defense office put the number of people missing at 80, most of them from the boats.

A coastguard rescue team continues to look for four people missing from the tugboat Tina, which capsized in the Tablas Strait between Mindoro and Romblon, said coastguard spokesman Lieutenant Armando Balilo.

Helicopters and surface craft from the Navy, Air Force, and local government units were searching for the fishermen, he added.

"There is no time (limit) for search and rescue. The bottomline is, we must be able to account for all of them," Balilo told AFP.

He said people have been known to survive for 10 days from shipwrecks in the tropical waters of the Philippines, "especially if they have provisions."

He was not ruling out the possibility some of the missing may have been taken aboard passing ships, although there have been no reports from these vessels.

President Gloria Arroyo, en route to Manila via the United States after attending a Pacific Rim summit in Chile and visiting Mexico, was monitoring the rescue, her spokesman Ignacio Bunye told local radio by telephone from Los Angeles.

"The president plans to visit the devastated areas as soon as she arrives" in Manila early Thursday, Bunye added.

Public works crews went about the slow task of clearing roads cut off by landslides and restoring power and telecommunications facilities in Aurora and Nueva Ecija a day after the tropical depression brought death and destruction to the two northern provinces.

The storm dissipated in the Cordillera mountain range overnight Tuesday, the weather bureau here said.

About 20 typhoons or storms hit the Southeast Asian archipelago every year, claiming an annual average of about 500 lives.

Civil defense official Neri Amparo told AFP that even though they are aware of the danger, subsistence fishermen continue to be among those most likely to die during the Philippines typhoon season that normally spans June to September.

"They put to sea despite the big waves because that is the source of their livelihood," she said.

These fishermen use small wooden boats with outriggers and powered by a small outboard motor, which she said "are not designed for rough seas."

Also, "because of the problem of poverty people live along the riverbanks, seawalls" and other vulnerable areas, Amparo said. About half of Filipinos live on two dollars a day or less.

"When the strong winds (or floodwaters) pass they are easily swept away," she said.

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