People are fleeing a surge of attacks in northern Mozambique where witnesses have described beheadings, mass kidnappings and villages burned to the ground, the United Nations said on Friday.

Officials said armed groups had stepped up assaults in Cabo Delgado province, the center of an Islamist insurgency that has killed hundreds since it started in 2017.

The northern region is also home to one of the world's biggest recent gas finds, where Exxon Mobil Corp, Total and others are working.

Displaced villagers have described killings, maiming, torture and destroyed crops, Andrej Mahecic, the spokesman for the U.N. High Commissioner for Refugees (UNHCR), said.

"They speak of men in particular being targeted and beheaded, and many, many reports of women and children ... being kidnapped or simply disappearing," he told a briefing in Geneva.

Some of the attackers appeared to be bandits. "But there is also the element of some of the groups being driven by ideological or other ideas. And they have been quite vicious ... in spreading the terror in this part of Mozambique," Mahecic added.

The UNHCR said there had been a sharp increase in violence in recent months, and the past weeks had been the most turbulent period since attacks began in October 2017. In all, 100,000 people have been uprooted by the violence in the last two years.

"In total, at least 28 attacks were carried out in the province since the beginning of the year," Mahecic said.

The militants - who tout their brand of Islam as an antidote to what they describe as a corrupt ruling elite - called themselves Ahlu Sunnah Wa-Jama when they started launching attacks in 2017.

More recently, Islamic State has claimed responsibility via its media outlets, though there has been no independent confirmation of a link.