Nigeria:Gunmen killed four Nigerian policemen in the oil-producing southern Niger Delta on Monday

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 traveling to Yenago, 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.

Residents have been long demanding a greater share of oil revenues. Crude oil sales account for around 70 percent of national income in Nigeria but there has not been much development in the poor Delta swampland.

President Muhammadu Buhari has extended a multi-million dollar amnesty signed with militants in 2009 to stop them attacking oil facilities but upset them by ending generous pipeline protection contracts.

Reuters