Shell Nigeria says armed militants attacked an oil flow station in the southern Delta and killed a guard, amid fears of a renewed crisis as the country grapples with halved petroleum prices.

Militant demands have been increasing and range from regional autonomy to a bigger share of the oil wealth that has polluted the Niger Delta region and destroyed the livelihoods of hundreds of thousands of farmers and fishermen.

President Muhammad Buhari has said he will terminate in December a $500 million-a-year amnesty program negotiated in 2009 for 30,000 militants that curbed an insurgency that was killing 1,000 people a year and halved the oil production of Africa's biggest producer.

Shell Nigeria spokesman Joseph Obari said Sunday that an investigation has been launched into Friday's attack at Kolo Creek.