Shell said on Thursday it had repaired and reopened a key pipeline in southern Nigeria shut down last week owing to leaks, but that contractual obligations remained suspended.

"We reopened the Trans Niger Pipeline (TNP) yesterday (Wednesday) after carrying out necessary repairs of the facility," Precious Okolobo, the spokesman for the SPDC, the Nigerian subsidiary of the Anglo-Dutch oil group, told AFP.

He said Shell had restored a daily oil production of 150,000 barrels per day.

But a force majeure declared in respect of Bonny Light exports following the closure would remain in force, he said.

Force majeure is a legal term releasing a company from contractual obligations due to circumstances beyond its control. Bonny Light is one of the main grades of crude produced in Nigeria.

The TNP has been repeated attacked by vandals and oil thieves with Shell claiming the pipeline has been closed down at least five times since early July.

Oil theft is a major problem in Nigeria, estimated to cost the country at least $6 billion (4.6 billion euros) per year in lost revenue.

Shell has consistently blamed its losses on thieves vandalising its pipelines.

However, environmental activists argue that Shell does not do enough to prevent such incidents and effectively clean up the damage when they do occur.

Nigeria's oil-producing region is badly polluted from decades of oil spills, and Shell is the biggest producer in the west African country, where it has been operating for over 50 years.

Nigeria produces some two million barrels a day of oil and is Africa's largest exporter.