The Peruvian government said Wednesday that 5,000 birds, mostly pelicans, and nearly 900 dolphins have died off the country's northern coast, possibly due to rising temperatures in Pacific waters.

The country's northern beaches were earlier this week declared off-limits as scientists scrambled to pin down what was causing such a massive toll, with non-government organizations blaming oil exploration work.

But Peru's deputy environment minister Gabriel Quijandria, disputed this and said warming waters, which disturbs species' food supplies, was a possible cause.

He said that 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 that the phenomenon "will extend to other coastal areas," Quijandria said, noting that 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 death of marine life is known.

One non-government conservation organization, 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 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 told AFP on Tuesday that the warming of the Pacific waters due to El Nino could be to blame.