The Niger Delta Avengers, a militant group that has carried out attacks on oil facilities in Nigeria’s southern oil region, said it would only negotiate with the government if independent foreign mediators were involved.

Last week the group rejected an offer from the government to hold talks.

Attacks in the southern swampland have pushed oil production to a 20-year low in what was until recently Africa’s biggest oil producing nation. Its production has fallen behind Angola in the last few weeks.

On Sunday, the Movement for the Emancipation of the Niger Delta, one of the region’s largest militant groups until it signed up for a government amnesty in 2009, urged the Avengers to take part in negotiations.