Nearly 900 dolphins and at least 5,000 birds, mostly pelicans, have died off Peru's northern coast, the government has confirmed.

The country's northern beaches have been declared off-limits as scientists scramble to pin down what has caused such a massive death toll.

Non-government organisations blame oil exploration work.

But Peru's deputy environment minister Gabriel Quijandria disputed that, saying rising water temperatures, which disturb species' food supplies, were a possible cause.

He said although tests conducted on 877 dolphins found dead on the coast had not been completed, contamination from heavy metals or the presence of bacterial infections was not responsible.

It is probable the phenomenon "will extend to other coastal areas," Mr Quijandria said, noting there could be a resulting increase in the numbers of birds and other sea life killed.

The South American nation's health ministry declared an alert at the weekend, urging the public to stay away from the beaches around Lima and on the northern coast until the cause of the deaths is known.

One non-government conservation organisation, known as ORCA, has blamed the dolphin deaths on oil exploration activities in the area, which it claims produces noises which are having an acoustic impact on the mammals.

A representative from the group, Carlos Yaipen, said on Wednesday it had tested 30 dead specimens and found broken ears and damaged organs consistent with the victims suffering "the bends", also known as decompression sickness.

Weather expert Abraham Levy said on Tuesday the warming of the Pacific waters due to El Nino could be to blame.

AFP