Authorities fear the grounded Greek tanker Tasman Spirit, could spill 5500 tonnes of crude it still has on board if its rear section breaks under the impact of rough seas.

"The ship has tilted to 18 degrees from 13 degrees on Saturday, because of the high tide. It is a very dangerous posture. At 20-plus degrees, the ship can break up," Brigadier Iftikhar Arshad, a senior official of the Karachi Port Trust said.

Carrying 67,500 tonnes of crude oil, Tasman Spirit had run aground on 27 July and spilled 25,000 tonnes of oil into the Arabian Sea, polluting the coastline and killing marine life.

Brigadier Arshad said chances of a further oil spill had increased, but authorities were trying to empty the ship "as soon as possible".

"It is a very dangerous posture. At 20-plus degrees, the ship can break up"

Karachi Port Trust Official

"We will hopefully complete the drainage operation by Monday but the operation is becoming dangerous with the passage of time," he said.

The oil spilled by the tanker has badly polluted a 16-km stretch of Karachi's eastern coastline.

Sea Angle, a small salvage tanker that arrived from the United Arab Emirates, is spearheading the delicate draining operation.

Environmentalists and doctors are already complaining of increased ailments such as headaches and nausea among residents of Karachi's beachside neighbourhoods.

With dead fish, turtles and sea snakes littering Karachi's coast covered by thick layers of crude oil, environmentalists say the damage caused by the spill has been huge.

They say it could take months to clean and restore the coastline.