At least 264 dead bottlenose dolphins have washed ashore over the past three days on Peru's northern coast, officials said Friday as they seek to discover what killed the marine animals.

The dead dolphins were found over a 103 kilometer (64 mile) stretch of sandy beach, said Edward Barriga, an official with Peru's Oceanic Institute (IMARPE).

"We have taken samples to determine the cause of death," said Barriga, speaking from the city of Lambayeque, adding that vast quantities of dead anchovies had also been found in the region.

The dolphins may have been killed by the impact of off-shore oil exploration and drilling in the region, said Carlos Yaipen with ORCA, a non-governmental group that focuses helping ocean creatures in the south Pacific.

The mass dolphin deaths are a "very serious" issue, Yaipen told AFP.

The head of a Lambayeque group representing aquafarmers, Jorge Cabrejos, said the anchovies appear to have eaten contaminated plankton, which then sickened the dolphins that ate the small fish.

Thirty-four of the world's 81 species of cetaceans swim off the Peruvian shores, 17 of which are dolphins. Of those, the most common is the bottlenose dolphin.