At least 85 dolphins have beached themselves in a shallow inlet of a US nature reserve at Cape Cod, officials said on Tuesday, adding that the cause of the mass strandings remained a mystery.

A spokesman for the International Fund for Animal Welfare said the huge number of beached mammals over the last two weeks, with most of them dying, was close to the amount usually recorded in the course of a year.

A female dolphin and her calf are carried back to the water. Credit:Reuters

"We had a total of 85 confirmed strandings since January 12 and that number might be as high 101," AJ Cady said. "There are still about 16 dolphins reported in difficult locations we haven't been able to confirm."

"Of that number 35 were still alive. Fifty of them were dead by the time we reached them," Cady said.