An environmental group says an oil spill off the coast of Nigeria this past week could stretch across more than 900 square kilometers of ocean.

The U.S.-based group, SkyTruth, says it estimates from satellite images it has obtained that the slick is 70 kilometers long and spans 17 kilometers at its widest point.

Royal Dutch Shell discovered the spill Tuesday about 120 kilometers off the southern coast of Nigeria.

Shell estimates the amount of leaked oil at less than 40,000 barrels and says the spill has been stopped.

Nigeria's National Oil Spill Detection and Response Agency says the amount of oil that leaked into the ocean is similar to the last major spill at a Mobil field in 1998.

Shell says the leak happened in a pipeline that transfers crude oil from a production vessel to a waiting oil tanker .

The company says it has deployed airplanes and vessels with dispersants to locate and break up the leaked

oil, which is moving towards the shore.