The Nigerian army has reported making three arrests against militants suspected of having been involved in the killing of four soldiers. During the arrest, a large weapons cache was seized, the army said on Tuesday.

Two weeks ago the army reported that gunmen disguised as priests had killed four soldiers in the oil-producing Niger Delta region. The area has witnessed nonstop violence and militant attacks on oil pipelines, according to Reuters.

Earlier Monday, the army claimed killing several militants and arrested three other suspects belonging to the Niger Delta Avengers group during a fierce exchange of fire. The group stands accused of being involved in the killing of the soldiers.

Large quantities of arms, ammunition and a gunboat had been seized, it said in a statement.

The Niger Delta Avengers group has claimed responsibility for a string of attacks on oil facilities to press for a greater share of oil revenues and development for the poor region. It did not claim the attack on the soldiers.

On Sunday a statement attributed to the Avengers said the group was ready for a ceasefire and a dialogue with authorities. Government officials have refused to comment on the ceasefire offer.

Pipeline attacks have reduced the OPEC member’s output by 700,000 barrels a day to 1.56 million bpd.