The bodies of around 400 dolphins washed up on beaches in northern Peru last month, baffling authorities who are so far mystified as to the cause, officials said Tuesday.

The mammals' remains were found on beaches stretching across the northern departments of Piura and Lambayeque, said Peruvian Sea Institute (IMARPE) official Jaime de la Cruz in Chiclayo.

Lambayeque is in far northern Peru, almost at the Ecuadoran border.

Samples of the animals' remains were sent to Lima for autopsies to be done, De la Cruz said.

The surge in deaths alarmed Carlos Yaipen, a marine veterinarian at the NGO Sea Animal Conservation Scientific Group (ORCA).

"We could again be seeing what happened in 2012 when there were 836 dead dolphins on beaches in the north," he warned.

Some local media reports speculated that toxic algae could have led to the deaths.

But Yaipen said if that were the case, then humans would have been affected as well when they ate sea animals from the area.

Yaipen said he wondered if a tragic reaction to nearby acoustic "impact" could have killed the dolphins, with oil companies doing exploratory drilling in the area.

His group took samples to have tests run abroad, he said.