Up to 1,000 tonnes of crude oil were being carried on the ship, the Mercury, which was lost on a crossing from Aktau in Kazakhstan to the Azeri capital, Baku.

A tragedy has occurred. More than 40 people have died

Ramil Usubov Azeri Interior Minister

More than 40 of the 51 people on board are feared drowned.

The BBC's Chloe Arnold in Azerbaijan says the slick is already reported to be 15 kilometres (10 miles wide) and 8km long.

She says some shipping industry officials have described the ship as totally unseaworthy.

Azerbaijan's state shipping company, Caspar, has sent a team of specialists to the scene to try to contain the slick.

Relatives were desperate for information

Environmentalists fear the leaking oil will cause an ecological disaster.

Rescuers battling against stormy weather to find the 41 missing people say they are unlikely to find any more survivors.

So far rescuers have found just nine survivors and recovered one body.

I have no information at all and there is nobody who can help me. They do not know anything

Bugar Guseinov Brother of missing man

Rescue efforts, hampered by high winds and driving rain, carried on through the night with five vessels searching for survivors.

Helicopters rejoined the search at dawn on Wednesday.

But there was thought to be virtually no chance that anyone else could have survived a night in the chilly waters.

"A tragedy has occurred. More than 40 people have died," Azerbaijan's Interior Minister, Ramil Usubov, said on Wednesday.

The ship went down in a storm on Tuesday morning after issuing a distress signal.

Some newspapers in Azerbaijan are reporting the ship sank because it was overloaded.

Azeri television said 15 stowaways may have been on board, along with the 43 crew and eight passengers.

Frantic relatives have gathered outside the Caspar office and the hospital to which the rescued people were taken, desperately seeking information.

Many were angry and in tears.

"My brother works on the ferry as a senior mechanic. I have no information at all and there is nobody who can help me. They do not know anything," said Bugar Guseinov.

Wind and waves

Caspar president Aydin Bashirov said the boat got caught in a storm in which winds of up to 65 mph (105 km/h) whipped the sea into six-metre (20 foot) waves.

He said the heavy seas caused oil containers on board the ship to shift to one side, sinking the vessel.

The President of Azerbaijan, Heydar Aliyev, has set up a commission to investigate the cause of the tragedy.

The sinking is the second sea disaster to hit Azerbaijan's shipping industry this year.

In July, six men died after their Azerbaijani tanker ship exploded in the Turkmenistan port of Turkmenbashi.