Nigerian authorities are trying to prevent the worst offshore oil spill in a decade from reaching the West African nation's southern coastline.

Officials said the spill from a Royal Dutch Shell oil field about 120 kilometers off the coast was moving toward the shore Thursday, two days after it began. The head of Nigeria's National Oil Spill Detection and Response Agency, Peter Idabor, says the amount of oil that leaked into the ocean is similar to the last major spill at a Mobil field in 1998.

European oil giant Royal Dutch Shell's Nigerian subsidiary estimates the amount of leaked oil at less than 40,000 barrels. The Bonga oil field where the spill happened can produce about 200,000 barrels of oil a day. The company has stopped production at the field indefinitely.

Shell Nigeria says about 50 percent of the leaked oil has dissipated due to natural dispersion and evaporation. It says the company has deployed airplanes and vessels with dispersants to locate and break up the remaining oil.

Shell Nigeria says the leak happened during a transfer of crude from a production vessel to a waiting oil tanker.

The company's onshore oil facilities in the Niger Delta region have leaked oil frequently in recent years. Shell blames the spills on sabotage by militants and oil theft.