The military says a tricycle taxi has triggered a homemade explosive buried near a military checkpoint that killed four civilians and a soldier in Nigeria's northeastern town of Biu.

Army spokesman Col. Sani Kukasheka Usman blamed Boko Haram Islamic extremists and said victims include a mother and her baby. He said Sunday's blast was caused by an IED buried long ago.

It's only the third explosion in the past three months in Nigeria, signaling the success of a military campaign that has forced the extremists out of towns and villages and has taken the battle to their Sambisa Forest strongholds. There, the air force has been bombing camps and the army reports near-daily victories including freeing thousands of captives.

The nearly seven-year-old insurgency has killed about 20,000 people.