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 has said.

UN officials said armed groups have stepped up assaults in Cabo Delgado province, where a rebellion by a group that espouses its brand of Islam as an antidote to what it describes as a corrupt ruling elite, has killed hundreds since it started in 2017.

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

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"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 on Friday.

While some of the attackers appeared to be bandits, Mahecic added that 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," he said.

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, many fleeing to islands with little infrastructure, it said.

"In total, at least 28 attacks were carried out in the province since the beginning of the year," Mahecic said. The violence has spread to nine of the 16 districts in the province, he added.

The fighters in the region called themselves Ahlu Sunnah Wa-Jama, when they started launching attacks in 2017.

More recently, the ISIL (ISIS) group has claimed responsibility via its media outlets, though there has been no independent confirmation of a link.

Calls for more troops

The northern region of Mozambique is also home to one of the world's biggest recent gas finds, where Exxon Mobil Corp, Total and other companies have set up operations.

In recent days, Exxon and Total have called for more troops to guard their facilities in the north, Reuters news agency reported.

Resources in the northern region of the East African country had been further strained by Cyclone Kenneth in April of last year, which left 160,000 residents in need of aid.

Infrastructure in the region has also been affected by recent floods, which have destroyed bridges.

Cyclone Kenneth struck just six weeks after Cyclone Idai devastated the central region of Mozambique.