By Kieran Guilbert

DAKAR (Thomson Reuters Foundation) - 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 United Nations official said on Tuesday.

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, U.N. 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," Lanzer said.

Boko Haram militants have killed about 15,000 people and displaced more than 2.6 million in a seven-year insurgency and they still launch deadly attacks despite having been pushed out of the vast swathes of territory they controlled in 2014.

Some of the displaced in the region have moved again due to fear of further violence or as a result of attacks by Boko Haram, two of which were recorded last month, according to the U.N. Office for the Coordination of Humanitarian Affairs (OCHA).

"We are very concerned about the security situation ... do not think that the crisis is anywhere near over," Lanzer told the Thomson Reuters Foundation by phone.

Chad earlier this year extended the state of emergency in the region, which has disrupted fishing, farming and cattle breeding and hit cross-border trade, markets and livelihoods, according to the U.N. World Food Programme (WFP).

Malnutrition rates in the lake region have dropped slightly over the past year as humanitarian access has improved, said Martha Nakakande of the International Medical Corps (IMC).

"But with people spread over so many different islands and IDP (internally displaced persons) sites, we are worried that we can't reach everybody," the IMC's nutrition officer said.

(Reporting By Kieran Guilbert, Editing by Katie Nguyen. Please credit the Thomson Reuters Foundation, the charitable arm of Thomson Reuters, that covers humanitarian news, women's rights, trafficking, corruption and climate change. Visit news.trust.org)