OIL deposits have been washed up along a 15-mile stretch of North Yorkshire coastline, with seabirds being killed and a wildlife organisation warning thousands more could be in danger.

Scarborough Borough Council said deposits of oil were found between Scarborough and Speeton late yesterday afternoon and overnight, with the largest being the size of a plate.

The oil was discovered at Scarborough South Bay, Cornelian Bay, Cayton Bay and at Filey overnight on Sunday and yesterday, although it is not yet clear where it came from. The council said this morning that the number of dead seabirds was believed to be no higher than 15, but it does not yet know what the wider impact on marine life may have been, although it said this afternoon that it believed the worst is over.

Yorkshire Wildlife Trust said up to 50 birds had been affected near Scarborough and puffins, guillemots, razorbills and shags could all be at risk. Staff at its Living Seas Centre at Flamborough are on "high alert" as thousands of birds return to the Yorkshire coast in preparation for breeding.

"The strong easterly winds and heavy sea conditions are thought to have assisted the oil's journey to the shoreline," said Andy Skelton, the council's head of environmental services.

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www.nch.com.au/ExpressDictate "The Maritime and Coastguard Agency has been trying to determine the source of the pollution, including conducting a surveillance flight, and, together with the Environment Agency, is arranging for samples of the substance to be tested."

He said surfers had raised the alarm yesterday after seeing deposits of oil at Cayton Bay and local surfing groups, the main users of the beaches at this time of year, had been advised to stay out of the sea until further notice. He said people were now surfing "without experiencing any apparent problems", but they should be cautious and let the council know if there were any problems.

Warning signs have also been put up as a clean-up operation, which will continue throughout today, gets under way, but beaches have not been closed. Dog-walkers have been urged to take care when using the beaches and report any oil deposits they find.

"Our staff have been out this morning inspecting the beaches which were affected yesterday to check for any further signs of oil," said Mr Skelton.

"Following these inspections, no further deposits have been brought onto the beaches following last night's tide and, together with our key partners, we take the view that the worst of the problems are over. However, our staff will continue to monitor our beaches and carry out any cleaning required should further deposits of oil be washed ashore.

"The RSPB and other wildlife organisations have received some reports of oiled seabirds. Our officers will monitor the situation during their beach inspectionsand we will maintain contact with the relevant organisations."

A spokesman for the Maritime and Coastguard Agency said the aerial surveillance had not detected any pollution and tests were now being carried out on one of the deposits.

Kirsten Smith, Yorkshire Wildlife Trust's North Sea Living Seas manager, said the organisation welcomed the "immediate action" taken by the council and other organisations, but said: "At this time of year, thousands of seabirds are returning from their wintering grounds and are starting to assemble offshore, ready for the breeding season.

"Oil or other harmful substances can be lethal to seabirds and the unfortunate timing of a spill like this could deal a devastating blow to Flamborough's celebrated seabirds."

She said birds which are affected by oil lose the "waterproof and insulating properties" of their feathers, preventing them from feeding and keeping warm, but they can be saved if they are found and cleaned. However, she said birds can try to preen the oil off their feathers and consume some of the oil, which can be deadly even if they are subsequently cleaned properly.

Anybody who finds a seabird in distress should contact the Living Seas Centre on 01262 422103 for advice.