Animal rescuers have began mopping up after an environmental disaster threatened to kill thousands of wild birds.

A major clean-up operation was launched when an estimated 10,000 litres of diesel oil leaked from a tanker in North Mundham, near Chichester.

The spillage oozed along countryside ditches before reaching Pagham Harbour nature reserve, home to a wealth of wildlife.

Despite desperate attempts to scare birds away from the slick, which has spread across the water surface, more than a dozen have been found dead and dripping in oil.

Animal rescuers, fearing for the thousands of birds that visit the reserve, have been working hard to capture the ones affected so they can be rushed to a RSPCA wildlife centre in Hastings.

Specialist cleaners at the centre were spending hours scrubbing the oil out of the birds' feathers and rehydrating them.

The Environment Agency has managed to prevent the disaster escalating by building a dam at the source of the spillage.

But the cost of the operation may reach £20,000.

Boons have also been used to stop the diesel spreading further.

Animal collection officer Marie Stevens, heading the RSPCA's efforts, has been using boats and agile hands to rescue the distressed wildlife caught unawares by the slick.

Without help, the oiled birds face a slow death as their clogged-up feathers fail to keep them warm.

She said: "We saved five geese and a moorhen but found several dead ducks.

"The geese were oiled but not as severely as the ones on Thursday when it first struck.

"First of all we rehydrate the birds, because the oil affects their feather structure and they get extremely cold and weak.

"Then they have a series of washes - we're using Fairy Liquid."

Ms Stevens said washing could be stressful for the birds and cleaners had to ensure they were fit enough to handle the process.

She said: "We've got bird-scarers in position to stop new birds flying down into the diesel.

"The oil emulsifies in the water and makes an oiling film across the surface.

"This can also kill fish because the water cannot aerate.

"The weather has been clear and still but we needed rain and wind to break up the oil."

Although many birds fly south for the winter, a host of different breeds still use the reserve in the colder months.

Ms Stevens said: "We saw about 300 geese flying in and began to panic but managed to scare them off.

"Hopefully, we'll contain most of the oil and keep the birds away.

"We are concerned about a number of ducks and mallards.

"This is a wild environment, not a local pond, and when you approach them they can go into the reeds and they are hard to see.

"Unfortunately, they will probably die.

"It can be a struggle to clean the birds but the cleaners do not get hurt. The birds give up once they know they cannot move."

The Environment Agency is still investigating the cause of the spillage and prosecutions could follow.

A spokesman said the clean-up was set to continue over the weekend and throughout next week.

Pagham Harbour, the smallest East Solent harbour, has about 11km of shoreline.

The nature reserve covers 1,000 acres and is one of the few undeveloped stretches of Sussex coastline.

More than 200 different species can be seen at the site, a natural breeding ground for birds.

They include little terns, ringed plovers, oystercatcher, lapwings, redshanks, reed and sedge warblers, reed buntings, little grebes, mallards, mute swans and moorhens.

The site was a harbour during the Middle Ages but the sea forced its way through sea defences and flooded the area during a storm in 1345.

A sea wall was built in the 19th Century to create another harbour but the sea broke through again in 1910.