About 250,000 gallons of oil spilled into bay waters off Rio de Janeiro early Tuesday, in what officials believe may be one of the country's largest ecological accidents.

The oil, which spilled from a ruptured pipeline running from a nearby refinery that is operated by the state-owned Petroleo Brasileiro SA, also known as Petrobras, had spread across a 24-square-mile area of Guanabara Bay by this afternoon. Petrobras initially reported that about 100,000 gallons of oil had spilled.

Miles of shoreline were black and slick with oil today as government officials began evaluating the damage done by the spill, which occurred about 3:30 a.m. Tuesday.

"It's a lot of oil, and the environmental impact is going to be great," said Andre Correa, the government secretary responsible for the environment in the state of Rio de Janeiro. "The situation is very grave."

Correa and other officials feared the effects of winds that, since Tuesday morning, carried the light oil across the bay toward an area inhabited by cranes, crabs, mussels, oysters and several species of small fish.

"The smaller fish, who like to swim very close to the water level, and the oysters and mussels--they're the ones who are going to be most affected," said Axel Grael, president of the state's environmental monitoring agency.

Correa said that the government plans to fine Petrobras about $26,500, a figure based on Rio de Janeiro's laws governing how companies should be punished for environmental accidents. And the company is also expected to pay for the environmental cleanup.

Since the accident, Petrobras has been accused of operating without required licenses and of neglecting to do periodic audits to determine the environmental impact of its work.

"Certainly the company has all the licenses it needs to operate," a Petrobras spokesman said this afternoon. "I'm not sure what they want. We do audits, every year, every month. It's more noise than oil that was spilled."

Guanabara Bay, among Brazil's most polluted bodies of water, is no stranger to spills. In 1997, an accident dropped 150,000 gallons of oil into its waters. A mishap in 1975 covered the bay with 1.2 million gallons of oil. Both accidents involved either ships working for Petrobras or the company's pipelines.