More than 4,000 people are being forced to flee their homes daily in Burkina Faso as attacks on civilians by armed groups increase in number and frequency, the United Nations has warned.

In a statement on Friday, the UN refugee agency (UNHCR) said more than 700,000 people were displaced in the past 12 months, with an estimated 150,000 of them being uprooted in the last three weeks alone.

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"People fleeing the violence report attacks on their villages by militant groups, killing, raping, pillaging. Terrified of these attacks, residents have left everything behind to find safety," the agency said.

Most of the displaced people were now living in host communities, but the UNHCR was finding it difficult to assist them because access was "problematic" in some regions due to the insecure situation, spokesman Andrej Mahecic told a news briefing.

Mahecic stressed that the situation in the Sahel region needs more attention, describing it as a "crisis" that is "poorly known and poorly understood".

Rise in violence

Burkina Faso borders Mali to the northwest and Niger to the east, with all three Sahelian countries hit by a swiftly deteriorating security situation.

Last month, the UN envoy for West Africa told the Security Council that attacks have increased fivefold in Burkina Faso, Mali and Niger since 2016, with more than 4,000 deaths reported in 2019.

Many parts of the Sahel that have seen the most fighting are severely underdeveloped. The multiple armed groups operating in the region, a semi-arid swathe of land beneath the Sahara, have exploited poverty as well as religious and ethnic divisions for recruitment. Meanwhile, the military campaigns by the ill-equipped national armies have also been marred by human rights abuses, which analysts say have pushed some civilians into the arms of fighters.

The rise in violence in the Sahel has fed a feeling of increased insecurity among locals.

The UNHCR said more than 4,400 refugees from Niger had arrived in Mali, fleeing a string of attacks in the Tillaberi and Tahoua regions, including an attack in early January.

The refugees had found safety in the Malian towns of Anderamboukane and Menaka, where they joined another 7,700 displaced Malians in the same area.

More people continue to cross the border between Niger and Mali, the agency said.

"Alarmed by the dramatic rise of forced displacement in the Sahel, UNHCR reiterates its call for the protection of civilian populations and those fleeing violence," the statement said, adding that "humanitarians need safe access to deliver assistance."