People in the affected areas are suffering nausea and headaches

Maritime experts say 12,000 metric tons of oil have spilled into the Arabian Sea while another 35,000 are still on board the Greek-registered Tasman Spirit.

Dead fish and turtles littered two main beaches and a key mangrove forest had been badly hit, environmentalists said, while doctors reported dozens of people suffering from nausea.

The authorities tried to play down fears of extensive environmental damage, saying the light quality of the crude meant it would evaporate quickly.

However, environmentalists have expressed concern about the ability of the authorities to cope with the oil spill.

Worldwide Fund for Nature spokesman Hammad Naqi Khan questioned whether the Karachi Port Trust and the relevant safety organisations had enough booms and pumping equipment for the job.

Fine threatened

A salvage operation is being conducted to remove the remaining oil before the vessel broke up completely.

The ship ran aground at low tide off the southern port city on 28 July and started spilling oil extensively this week.

Pakistani authorities have brought in a C130 aircraft from Singapore to spray chemicals along the affected coastal area, while other equipment is being used to minimise the damage to the beaches.

MAJOR TANKER SPILLS 1978 Amoco Cadiz Brittany - 220,000 tonnes 1979 Atlantic Empress Tobago - 160,000 tonnes 1967 Torrey Canyon UK - 119,000 tonnes 1983 Braer Shetland Isles - 85,000 tonnes 1996 Sea Empress Wales - 72,000 tonnes 2002 Prestige North-west Spain - 42,000 tonnes 1989 Exxon Valdez Alaska - 38,800 tonnes In pictures - Pakistan oil spill

Brigadier Iftikhar Arshad, general manager of the Karachi Port Trust, said tug boats were being used to hold together the vessel while the oil still on board was taken off.

He said there was a danger that pipes on the ship might rub against each other and cause sparks that might blow the vessel up.

However, the trust was confident the spillage had been stopped.

The owners of the vessel, who are bearing the cost of the clean-up, said the Iranian light crude could be easily dispersed.

Greece-based Polembros Shipping said 20,000 tonnes of the 67,000-tonne cargo had been removed from the ship.

"Given the cargo involved, it is anticipated that the clean-up will proceed rapidly and with little long-term impact," the company said in a statement.

However, a thick layer of oil was reported to be covering much of the coastline around Karachi and port authorities have threatened to fine the owners 10 million Pakistani rupees ($173,000).

TV crew treated

Paramilitary troops have been called out to cordon off one of the main beaches near an upmarket residential district.

IUCN - The World Conservation Union says 16 kilometres (10 miles) of the Arabian Sea coast has already been polluted.

The Tasman Spirit still has 35,000 tonnes of crude on board

The IUCN's Tahir Qureshi said: "This is an ecological, environmental and economic disaster. Oil spillage always has a long term and short term effect."

Residents near the sea have also been complaining of feeling unwell and a television news crew were taken to hospital on Friday after inhaling toxic fumes while filming on a beach.

Dr Wafay Shakir, a neurologist at a Karachi hospital, told the Associated Press that exposure to crude oil "may cause nausea, headaches and throat infections".

The authorities say they could not have done more to deal with the disaster, and that Karachi residents are not in danger.