A fourth vehicle exploded, apparently set off by a suicide bomber.
Fifteen civilians and six suicide bombers died while five soldiers and 12 civilians were wounded, said Defense Ministry spokesman Brig. Gen. Chris Olukolade.
A surviving suicide bomber was arrested, he said. Eight vehicles were destroyed in addition to the ones carrying explosives.
"Three of the four explosive-laden vehicles were demobilized by shots fired at them by soldiers at the checkpoint shortly before the explosions that rocked the area," Olukolade said in a statement. He said the four vehicles appeared headed for the oil depot.
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Three blasts damaged the perimeter fence of Peace-FM radio station located opposite an oil depot and fuel station of the state-owned Nigerian National Petroleum Corp, said a security official who spoke on condition of anonymity because he is not permitted to give information to journalists.
It is located at Mulai, just 5 kilometers (3 miles) outside Maiduguri, the capital of Borno state that is the birthplace of the Boko Haram terrorist network.
On March 14, Islamic extremists attacked the main military barracks in Maiduguri and freed hundreds of detainees. The military said it killed scores of freed detainees.