Gunmen killed four Nigerian policemen in the oil-producing southern Niger Delta on Monday, police said.

The officers were ambushed in the Okobie community while travelling to Yenagoa, the capital of Bayelsa state, police spokesman Ahmad Mohammad said.

He said it was unclear who was behind the attack.

Attacks on oil pipelines and kidnappings have been rising in the Delta, which produces much of Nigeria’s oil output, since authorities issued an arrest warrant in January for a former militant leader on corruption charges.