More than two million people have been forced to flee their homes within their own countries' borders owing to violence engulfing Africa's Sahel region, the United Nations said on Friday.

The humanitarian response is "dangerously overstretched" in an area covering parts of Burkina Faso, Chad, Mali and Niger, said UNHCR, the UN Refugee Agency.

Thousands have died since jihadists launched an insurgency in northern Mali in 2012 that spilt over its borders.

UNHCR spokesman Boris Cheshirkov told reporters in Geneva the "unrelenting violence" must stop.

He siad the figure of two million internally displaced people had more than quadrupled since the start of 2019.

"The extreme vulnerability of the Sahel has been laid bare by the impact of forced displacement, caused by widespread and gruesome violence perpetrated by armed insurgent groups and criminal gangs," he said.

"The humanitarian response is dangerously overstretched, and UNHCR is urging the international community to redouble its support for the region."

He said the countries needed help particularly with schools and hospitals, many of which have shut because of the violence.

More than half of the displaced in the region are in impoverished Burkina Faso, which has been under attack since 2015 when Islamists swept in from neighbouring Mali.

The problems in Burkina Faso have intensified in recent weeks, said Cheshirkov.

More than 11,000 people, mostly women and children, fled attacks in and around the northern town of Koumbri earlier this month, he said.

They have reached safety and the UNHCR is erecting shelters and distributing aid.

"Despite the generosity of their hosts, many of the IDPs lack basic shelter and are sleeping under open skies," said Cheshirkov, adding that they urgently needed essential supplies and facilities to stop Covid-19 from spreading.

On top of the internally displaced, more than 850,000 people have fled from Mali and taken shelter in other countries.

Throughout the Sahel, UNHCR and other agencies are working to help hundreds of thousands of displaced people with shelter, aid and cash, the spokesman said.

"Our teams are also working to prevent and respond to instances of sexual violence, which have become widespread," he added.

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