Nigeria's state oil company is shutting down two of the country's four refineries because militant attacks on pipelines have affected their supplies.

A statement Wednesday announced an "operational shutdown" of refineries in Port Harcourt, the oil capital in the southern Niger Delta, and in northern Kaduna city after weekend attacks on two strategic oil supply lines.

The refineries had recently reopened following months of inactivity. Oil-rich Nigeria imports 90 percent of refined products and suffers regular fuel shortages because of a lack of maintenance.

The weekend attacks came after a court ordered the arrest of a former warlord accused of corruption. The Niger Delta had been relatively peaceful since a 2009 amnesty halted attacks by militants demanding a bigger share of revenues from oil production that has destroyed the environment.