A Shell pipeline taking crude oil to a processing station in Nigeria’s Rivers state was closed after a leak was detected, the company said on Monday, but the cause was not yet clear.

“We discovered a leak on a line that connects an oil well with the Nembe Creek 2 flowstation on August 29,” Shell spokesman Precious Okolobo said.

“We have shut down the affected line to prevent any impact on the environment,” he said, adding that an investigation was underway to determine the cause of the leak.

He said a team of engineers had been mobilised to fix the damaged pipeline and that the impact on production was not considered major.

Nigeria’s oil-producing Niger Delta region has been badly polluted by decades of spills due to a range of factors, including sabotage by thieves or militants as well as operational or maintenance faults by oil firms.

Activists say companies such as Shell have not done enough to prevent leaks.

Nigeria is Africa’s largest oil producer, with output at more than two million barrels per day. Shell has been the country’s biggest producer.