Oil giant Royal Dutch Shell has said acts of sabotage are increasing in southern Nigeria.
The Anglo-Dutch company said it had suffered at least three separate incidents of sabotage on its pipelines in the Niger Delta this month.
In a statement, Shell said it had put containment booms into surrounding waterways to stop the flow of oil.
Thieves known as bunkerers often try to pierce the pipes to siphon off oil and sell it on.
Shell official Babs Omotowa said thieves had drilled holes or used hacksaws to pierce pipelines in the Cawthorne Channel leading to the terminal at Bonny in Rivers State.
"The environmental and social cost of widespread sabotage is simply unacceptable," he said.
The Niger Delta is a vast network of mangrove creeks that make up one of the world's largest wetlands.
Shell said it had set up containment booms to stop crude oil flowing further into the mangrove swamps.
Poor communities in the Delta have suffered decades of environmental pollution from oil spills.
Militants have also carried out repeated attacks against oil installations and pipelines.
Shell says it paid $4m (£2.6m) in compensation last year related to oil spills in Nigeria.