The ship went down on Friday in rough seas in the Panay Gulf between the central islands of Panay and Guimaras. Eighteen of the crew were rescued but two remain missing, the coastguard said.

"We don't have the capability right now to salvage sunken vessels this deep. That's why we're seeking international support," said coastguard spokesman Lieutenant-Commander Joseph Coyme.

The coastline of Nueva Valencia, a quiet fishing town on the southwestern coast of Guimaras, has borne the brunt of the disaster with black slime coating the coastline.

Provincial governor Joaquin Carlos Nava told reporters: "These people cannot clean up this mess by themselves. They need international support."

Officials said the government had asked for equipment and specialist teams from Japan as well as a team from Indonesia to help manage the slick, which is around 460 kilometres (280 miles) south of Manila.

Coyme said the challenge was to contain the 19.5 nautical-mile-long slick off the southern coast of Guimaras and to plug the leaking tanker, which is resting on the seabed in around 3,000 feet (900 metres) of water.

Local crews have attempted to put booms around the spill to keep it from spreading but their work has been severely hampered by rough seas. The coastguard also appealed for help in trying to salvage or refloat the ship.

The slick has hit more than 200 kilometres of coastline, damaging mangrove swamps, seaweed plantations and coral reefs containing popular dive sites, governor Nava said.

The international environmental group Greenpeace called the sunken vessel an "ecological time bomb that may cause long-term and possible permanent damage to the environment and livelihoods of people."

It urged Manila in a statement to hold the tanker owners and its charterers "accountable for damages to marine and coastal ecosystems and for their rehabilitation."

Coyme estimated the spill, described by the coastguard as the worst in the history of the Southeast Asian country, would take more than a year to clean up.

The slick could eventually threaten the west coast of the island of Negros as well as the eastern flank of Panay island.

Of particular concern for conservationists is damage to the Taclong island national marine reserve off the south of Guimaras.

The network of three species of mangroves, reefs of nine types of hard coral and beds of seven types of seagrass serves as a "feeding, breeding and nursery ground" for 144 fish species, the coastguard said.

It said the spill was "wreaking havoc" on fishing grounds and other coastal areas.

However Coyme said the slick was unlikely to reach the resort island of Boracay off Panay's northern coast. The island is the Philippines's top tourist attraction and is famed for its white sand beaches.

Governor Nava said that in his province alone up to 10,000 fishermen and their families were being affected by the disaster.

"It is not only the coastline and fishing industries that are affected but also the tourism industry," he said.

"We are looking at the possible evacuation and support for our displaced fisherfolk. We don't know how long this (clean-up) will take," he told ABS-CBN television.

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