Just when Nigerians have started forgetting the ugly tales of Boko Haram insurgency, they make a grand return.

On Wednesday, the cut-throat terrorists launched a fierce attack on the city of Maiduguri in Borno State.

This is the belligerent group’s most serious assault on the city in about a year and a half.

The raid comes six months after President Muhammadu Buhari said Boko Haram had "technically" been defeated by a military campaign that had pushed many of the jihadists deep into the remote Sambisa forest, near the border with Cameroon.

Aid workers and witnesses told Reuters News agency that they heard explosions and heavy gunfire for at least 45 minutes in the southeastern and southwestern outskirts of Maiduguri. Thousands of civilians fled the fighting, according to the Reuters witnesses.

But Nigeria's military has said it has everything under control, noting that it had contained the attack.

"The situation in Maiduguri is under control," it said in a statement, urging Maiduguri residents not to panic.

Fighter jets roared overhead as soldiers and police sped towards the scene, the Reuters witnesses said. Three children were hit by bullets, one witness said.

But despite the military's success in liberating cities and towns, much of Borno remains off-limits, hampering efforts to deliver food aid to nearly 1.5 million people believed to be on the brink of famine.