More than 100 birds have died in the San Francisco Bay area after their feathers became coated with a mystery "goo" resembling rubber cement that wildlife officials say has them mystified.

The mysterious substance has been polluting the waters on the eastern side of San Francisco Bay, fouling hundreds of sea birds and killing many of them, state wildlife officials reported.

"We've never seen anything quite like it," said California Department of Fish and Wildlife spokesman Andrew Hughan. "So it's a mystery for the moment."

Necropsies and lab tests are being conducted but results may not be available until later this week, he said.

Initial field tests of the goo when it was first spotted last week came back negative for petroleum, with officials saying they hope further comprehensive laboratory tests may identify the substance.

In the birds that died, the colorless, odorless slime damaged the ability of their feathers to insulate them in the cold weather and they succumbed to hypothermia, the officials said.

Dead birds have been found on beaches, shorelines and area trails. Volunteers have helped capture and clean some 300 shore birds of several species, hoping to return them to the wild.

"This has been incredibly difficult and taken a lot of time per bird," says Rebecca Dmytryk of Wildlife Emergency Services, one of the volunteer groups taking part.

Officials said they were investigating whether the substance could be the industrial chemical polyisobutylene, used in producing synthetic rubber, which was blamed in the death of thousands of seabirds in the United Kingdom in 2013.

"While on its face, this substance seems very similar to reports from the U.K. two years ago, we won't know definitively until lab tests are completed," says Barbara Callagan of International Bird Rescue.

"The goo appears to be light gray in color and to me looks like rubber cement that's been played with all day and is sort of dirty," she says. "It has very little smell."

It is most likely a man-made substance, she adds, meaning it could have come from a burst pipeline or might have been intentionally dumped by someone.

The ongoing contamination of the birds suggests the substance is being slow to dissipate in the waters of the bay, Hughan says.

"It was thick enough to see in the water for a few days and now you can't really see it unless you know where to look," he says. "It's a real mystery. We've never seen anything like it and neither have the bird rescuers."

It is not a public safety or health risk to humans, officials said.