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About 500 of tonnes leaked from underwater pipes at the Petrobras refinery in the picturesque Guanabara Bay early on Tuesday causing a slick of 40 sq km.

Environment secretary Andre Correa described the leak "extremely serious, regrettable and irreversible" as oil began to wash up on beaches near Rio.

"We are seeking the maximum punishment for this," Mr Correa said. "It was the second largest accident in terms of environmental impact."

Guanabara Bay is already heavily polluted and was the scene of a 6,000 tonne leak from an oil tanker in 1975.

Mr Correa said Petrobras would be asked to take part in the clean-up. He said the maximum penalty for polluting the environment was a "ridiculously low" sum of $23,000.

The oil has washed up on two small beaches, a mangrove swamp and part of the popular resort of Paquetra island.

Offshore

The leak occurred about 20m into a 17km-long underwater pipeline taking oil from Petrobas' Reduc refinery at Duque de Caxias to an offshore tanking terminal.

Mr Correa said of most concern now was the strength and direction of the prevailing wind, which has already pushed the slick closer to beaches.

Rio's famous Copacabana and Ipanema beaches lie a long distance away and should not be affected, he said.

Floating barriers are being used to concentrate the slick, after which sucking devices will be used to reduce the volume gradually for dumping back at Reduc's storage tanks.

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