US energy giant Chevron said Tuesday it had successfully sealed a ruptured offshore well which for the past week has seeped oil into waters off Brazil's Rio de Janeiro state.

The company "can currently advise that well control operations have been successful and that any fluid flow from the well appears to have ceased," a Chevron statement said.

"Monitoring also indicates a significant decrease in the amount of oil observed coming from nearby seep lines on the ocean floor. Chevron Brasil will continue to monitor the situation during the plugging and abandonment process in the coming days, which will culminate in the final cementing of the well."

The well, located about 1,200 meters (4,000 feet) underwater, had been leaking crude at the rate of some 400 barrels per day, according to state officials quoted by the news agency Agencia Brasil.

Chevron devised a plan, approved by Brazil's National Petroleum Agency, to encase the well in cement, ensuring that it will never be accessed again. The well has been leaking since last Tuesday, the company said.

The company said on Monday that the mishap at its Frade project some 370 kilometers (230 miles) northeast of Rio de Janeiro has created a widening oil slick, prompting the cessation of drilling activities.

It added that 17 vessels were working on a rotational basis to deploy containment booms and undertake skimming and washing techniques aimed at controlling a sheen that has slickened the ocean surface.

Brazil's President Dilma Rousseff has demanded a "rigorous investigation into the causes of the accident."

There was no immediate official word on the environmental impact of the spill.

Brazil in recent years has been moving to tap huge reserves of oil and gas discovered in very deep water -- at depth up to 7,000 meters -- in the Atlantic, under a thick layer of salt, requiring huge investment.