Transpetro, a pipeline subsidiary of state oil company Petrobras, said it was “a victim of criminal acts through the stealing of oil and derivatives” and was cooperating with authorities in their investigations.

It said 60,000 liters (16,000 gallons) of oil had leaked into Estrela River feeding into Rio's Guanabara Bay.

Transpetro said it repaired the pipeline and mobilized 400 people in response, and it had cleared 75 percent of the spill.

Mauro Moscatelli, a biologist who has long pressed authorities to better protect Rio's environment, said that while the scale of the spill was “medium” it would be “very serious” for flora and fauna.

“The big problem is when oil enters into the soil of the mangrove forests which are extremely important for maintaining biodiversity” in the area, he said.

“We'll know in the next two weeks the extent of damage to the trees and the fauna that will die -- especially crustaceans that are in their reproductive season right now.”

Moscatelli flew over the affected area on Sunday, and said at the time the spill was four kilometers long in the Estrela River.

He also said locals had noticed the spill on Friday, and he questioned why it took Transpetro so long to identify and react to it.

Guanabara Bay, a large bay along which part of Rio de Janeiro is located, has been badly impacted by pollution and urbanization.

It has suffered three major oil spills in the past, the last being in 2000 when 1.3 million liters (340,000 gallons) leaked from an underwater pipeline to a Petrobras refinery, decimating marine life and mangrove swamps.