Up to a million people around West Africa's Lake Chad are cut off from humanitarian aid by Boko Haram despite a regional military offensive against the Islamist militants, a UN official said today.

Boko Haram violence has uprooted more than 100,000 people across the swamplands of Lake Chad, where the borders of Chad, Cameroon, Niger and Nigeria meet, and disrupted the livelihoods of hundreds of thousands of others, according to aid agencies.

Security sources say a regional task force is wrestling back control of the lake – where Boko Haram controls part of the fishing industry in a labyrinth of waterways – with hundreds of militants having surrendered in the past month.

Yet many areas are impossible to reach amid the insecurity, said Toby Lanzer, UN humanitarian coordinator for the Sahel.

"We believe that there are up to a million people in the areas and villages we haven't been able to reach," he said.

Boko Haram has been active since 2009 and militants have killed an estimated 20,000 people over the past seven years. More than 910 schools have been targeted by the group, whose name means "Western [or non-Islamic] education is a sin". At least 611 teachers have been deliberately killed and another 19,000 forced to flee. At least 1,500 schools have closed.

Last week, the UN warned that 75,000 children are at risk of dying within "a few months" due to a famine caused by Boko Haram's insurgency. Some territory has been reclaimed from the group, but an estimated 2.6 million people remain displaced.

Upon his election last year, President Muhammadu Buhari pledged to stamp out the militants but has as yet failed to do so.

Additional reporting by Reuters.