More than 200 dolphins have beached themselves on Manila Bay, officials in the Philippines said as they tried to work out why the marine mammals had come ashore.

Residents saw huge pods of dolphins near the towns of Pilar and Abucay west of Manila, and Bataan Governor Enrique Garcia said at least three have died.

"This is an unusual phenomenon," Bureau of Fisheries and Aquatic Resources director Malcolm Sarmiento told local radio, estimating the number of dolphins at "more than 200."

He said they could be reacting to a "heat wave or disturbance at sea", such as a possible major underwater earthquake.

Since they are mammals, the dolphins have ears that are sensitive to large changes in pressure underwater, he said.

"If their eardrums are damaged they become disorientated and they float up to the surface," Mr Sarmiento said.

Mr Sarmiento said authorities' first concern was to keep the dolphins alive, and experts are being summoned to the area to help.

He said smaller pods of dolphins numbering "in the tens and twenties" had beached themselves elsewhere in the Philippines previously, but this was the first time so many had done so at the same time and place.

-AFP