Port Harcourt, Nigeria ? Militants battling for a bigger cut of Nigeria’s oil revenue exploded a car bomb inside a military base, ending weeks of relative calm with a blast that killed at least two people and sent a fireball into the night sky.

The Movement for Emancipation of the Niger Delta claimed responsibility for the blast late Wednesday at a barracks in the southern oil center of Port Harcourt.

The group’s attacks on oil installations in Nigeria – Africa’s leading oil exporter – already had helped drive up world oil prices. The country is the fifth-biggest source of U.S. oil imports.

But the movement had not carried out any major attacks since February and never used a car bomb. Its previous military targets were small military boats.

Attacks on oil installations this year in the main oil-producing Niger Delta claimed by the movement have cut more than 20 percent of Nigeria’s daily oil exports of 2.5 million barrels.