Researchers have determined something other than a viral epidemic killed thousands of North Sea seals, but a government spokesman making the announcement Friday refused to give the cause.

Officials in London said seals had begun to die in British waters and estimated more than 10,000 of the marine mammals have died in recent months.If accurate, that estimate would represent a major increase in the number. Scientists elsewhere in Western Europe have estimated the number about 3,000 lower.

Animal health experts and environmentalists had speculated chemical pollution in the water may have lowered the seals' ability to fight disease.

The Dutch National Institute for Public Health and Environmental Protection has determined what caused the seal deaths and is working on a cure, according to a Health Ministry official who spoke Friday on condition of anonymity. She said the researchers would disclose their findings next week.