Gunmen killed 12 police officers in weekend attacks targeting security agents in two of Nigeria’s oil-producing states in the Niger River Delta in an escalating conflict fueled by secessionist tensions.

Seven of the officers were killed in Rivers State in the southern part of the country, the police confirmed in an emailed statement to Bloomberg Sunday.

Authorities have blamed previous incidents on the vigilante arm of the Indigenous People of Biafra, which is seeking to break away from the country.

Usman Umar in Abuja has more in this report.