Brazilian police are investigating an oil spill in an offshore field operated by the US company Chevron.
Ships are working to disperse the slick 120km (75 miles) off the coast of Rio de Janeiro state, and Chevron says it has plugged the oil well.
Brazil's Energy Minister Edison Lobao has said the company will be "severely punished" if it is found to have failed in its environmental responsibilities.
In recent years Brazil has discovered huge oil reserves in the Atlantic.
The oil is leaking from a well in the Frade oil project, 370km (230 miles) off the Brazilian coast.
Chevron initially estimated that 400-650 barrels of oil had formed a sheen on the water after seeping from the seabed near the well.
But the international environmental group Skytruth said satellite images suggested the spill was many times bigger.
Brazilian energy minister Edison Lobao said the spill "was not as serious as had been announced" and was not moving towards the Brazilian coast.
But he said Brazil's oil agency ANP was monitoring the situation closely and would apply the full force of the law.
"If Chevron is not fulfilling its responsibilities, it will be more severely punished," he said.
ANP said underwater images showed Chevron's effort to permanently seal the well with cement appeared to have been successful, although there appeared to be a residual flow of oil from the seabed.
"The slick is continuing to move away from the coast and dispersing, as is desired," it added.
'Bad faith'
Police environment experts have been sent on navy helicopters to assess the scale of the spill.
Green Party members of the Brazilian Congress have called for a debate on the matter.
Federal deputy Jose Sarney Filho said Chevron appeared to have underplayed the scale of the accident.
"What has alarmed us is the lack of transparency on the part of the company and the attempt to minimise the size of the disaster," he told the official news agency Agencia Brasil.
"This is a clear demonstration of bad faith," he added.
Chevron said on Thursday the flow of oil from the ocean floor has been reduced to "infrequent droplets" and the remaining oil sheen on the surface was estimated at less than 65 barrels.
"Chevron continues to fully inform and work with Brazilian government agencies and industry partners on all aspects of this matter," the company said in a statement.
In recent years Brazil has discovered billions of barrels of oil in deep water that could make it one of the world's top five producers.
So far there has been little public debate about the environmental dangers of offshore drilling.
Political discussion has instead focused on how future oil revenues should be divided between different states.