Manila: A sunken tanker off Manila was leaking "minimal" oil, the Philippine Coast Guard said Monday, as a salvage company prepared to offload the vessel's 1.4 million litres of industrial fuel cargo.

The tanker sank in bad weather off Manila early Thursday, killing one crew member and leaving the Philippines facing the possibility of its worst oil spill ever.

Divers began sealing the vessel's leaking valves on Saturday, reducing the amount of oil flowing into the water to one litre per hour from 7.5 litres per minute.

The leaking was now "minimal scale" and "very controllable", said Lieutenant Commander Michael John Encina from Bataan Coast Guard Station.

The tanker went down as heavy rains fuelled by Typhoon Gaemi and the seasonal monsoon lashed Manila and surrounding regions in recent days.

An aerial inspection of Manila Bay on Monday found the oil slick from the tanker had dramatically reduced in size to 3.2-6.4 kilometres (2-4 miles).

On Saturday it had been 12-14 kilometres, Encina told reporters.

Offloading the 1.4 million litres of industrial fuel oil from the tanker, which was resting on the sea floor under 34 metres of water, was expected to start on Tuesday and would take around 10 days to complete.

Encina said the contracted salvage company would initially transfer 300,000 litres of industrial fuel oil to two other vessels.

That should be sufficient for the tanker to float and be towed to a site where the rest of the cargo can be removed.

The leaking valves of a second tanker that sank in Manila Bay have also been sealed, the coast guard said.

The Philippines has struggled to contain serious oil spills in the past.

It took months to clean up after a tanker carrying 800,000 litres of industrial fuel oil sank off the central island of Mindoro last year, contaminating its waters and beaches and devastating the fishing and tourism industries.

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