Royal Dutch Shell PLC has issued a production warning over its operations in Nigeria as it says thieves continue to cut into its pipelines, causing oil spills and damage.

Shell said in a statement Tuesday that it was declaring "force majeure" on shipments of Bonny Light crude oil after it shut down its Nembe Creek Trunkline, a major pipeline for the company. That means the company cannot deliver the promised supply from the field.

On Monday, the company issued a statement saying its Nigerian subsidiary lost some 150,000 barrels of oil a day over several days in late February after safety systems shut down the trunk line. Shell said the shutdowns came after thieves cut into the line.

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