An oil spill from a leaking well off the Brazilian coast northeast of Rio de Janeiro involving US oil giant Chevron "is not over," the National Oil Agency warned Sunday.

"The leaking still has not stopped at some points," the monitoring agency said in a statement, referring to images taken Saturday, and data from the Navy "which monitored 400 m (1,312 ft) of cracking" early Sunday, the G1 news service reported.

The oil "continues to drift away from the coast," the report added.

On Saturday, Chevron Brazil president George Buck told local media "the pressure of the deposits was underestimated."

Based on an incorrect calculation, the company used a type of material that lacked sufficient weight needed to contain oil, which then leaked and reached the surface, Folha de Sao Paulo newspaper explained.

Buck had insisted the leak was fully contained on November 13, but noted that the incident was still being investigated.

The oil agency said Friday that the oil slick was down from 163 square kilometers (63 square miles) on Tuesday to about 18 kilometers (11 miles) long and 11.8 square kilometers (4.5 square miles) wide.

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.

According to Chevron, the slick reached a volume of 882 barrels of oil at its worst, on Monday, and dropped to 18 barrels on Friday.

The US oil giant says no oil leaked from the wellhead, and that it was controlling and monitoring the "sheen."

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.