Royal Dutch Shell PLC says a major pipeline it has in Nigeria has come under repeated attack by thieves wanting to steal the crude oil it transports.

In a statement Monday, Shell says its Nigerian subsidiary lost some 150,000 barrels of oil a day over several days in late February after safety systems shut down its Nembe Creek Trunkline. Shell said the shutdowns came after thieves cut into the line.

Shell said its workers saw "large swathes" of water and land pollution from the oil spilled. The oil giant said it replaced this pipeline in 2010 at a cost of $1.1 billion.

A 2009 amnesty program reduced militant attacks on oil facilities, but oil theft has since increased.

Nigeria is a top supplier of crude to the U.S.