Royal Dutch Shell PLC says it is conducting reparations after detecting a leak blamed on oil thieves in Nigeria's oil-stained southern delta.

A Shell spokesman said on Tuesday evening that its Nigerian subsidiary has cut production by 70,000 barrels per day and isolated its damaged Nembe Creek trunkline in Bayelsa State.

Spokesman Tony Okonedo said an investigation showed oil thieves had installed valves at two different points and that more than 200 barrels of spilled oil have been recovered.

The discovery on December 24 came days after Shell apologised for another leak that caused Nigeria's worst offshore spill in more than a decade.

Shell's pipelines run across Nigeria's resource-rich region of swamps, mangroves and creeks. Nigeria is a top supplier of crude oil to the US.