US energy giant Chevron drew fire from Brazilian officials Friday amid conflicting accounts of the size of an oil spill at an offshore well it operated off Rio de Janeiro state.

Rio state's Environment Secretary Carlos Minc questioned official accounts of the spill, saying the "accident must be bigger than what is being announced."

"We are going to demand compensation for (the damage done) to birds, fish, dolphins and fishermen," he told Globo television. "The company must be punished in an exemplary manner."

The well, which Chevron said began leaking on November 8, is near the Frade field located some 370 kilometers (230 miles) northeast of Rio de Janeiro, in an area that is a migratory route for whales and dolphins.

The National Petroleum Agency said an overflight of the area and underwater images showed that the spill has been "substantially reduced" since Wednesday when Chevron began a cementing operation to plug the well.

The agency said the oil slick "continues to move away from shore" and estimated that it was now 11 miles (18 kilometers) long and 4.5 miles square (11.8 kilometers square), down from 63 square miles (163 square kilometers) on Tuesday.

It estimated that between 200 and 330 barrels of oil have seeped into the sea since November 8. The energy ministry, for its part, said 220 to 230 barrels of oil were seeping into the ocean daily.

Those estimates were contested by Greenpeace, which said satellite pictures showed a spill "10 times bigger," and likely reached closer to 3,700 barrels a day.

Another environmental group, SkyTruth, said the most recent images it could obtain on November 12 show an oil slick spreading from the drilling location over an area of 2,379 square kilometers (920 square miles), with an estimated volume of 14,954 barrels of oil.

If the spill began midday on November 8, 24 hours before the group first observed it on satellite images, the spill rate would be at least 3,738 barrels per day -- again more than 10 times larger than Chevron's estimate -- according to SkyTruth.

Chevron did not immediately return requests for comment.

"There is no doubt that an offense occurred. The spill results from the drilling activity," said Fabio Scliar, head of the federal police's environment department. "What interests me now is determining the responsibilities."

Federal police are looking into the case.

Greenpeace activists spilled black ink on the door of Chevron's Rio office to protest the US energy giant's drilling off the Brazilian coast.

"Chevron: your trash, our problem," read a placard waved by an activist outside the Chevron office in central Rio.

The activists said they wanted more clarity on the cause of the spill.

They also wanted more details on Chevron plans to control the spill and reduce its impact on the biodiversity of the Rio state coast, a migration route for dolphins and various species of whales.

On Thursday, Chevron said cementing operations were underway "as part of its well plugging activities on an appraisal well located in the vicinity of the Frade field offshore Brazil."

"There has never been any oil flow from the wellhead and current monitoring indicates oil from nearby seep lines on the ocean floor have reduced to infrequent droplets," it added, saying it was monitoring the "substantially dissipated" oil sheen.

Greenpeace called for more "transparency" from Chevron and government agencies, saying they had provided "contradictory" information.

"The company is playing down the problem," said Leandra Goncalves of Greenpeace, warning the oil slick could exceed 160 square kilometers (62 square miles).