Violence in northwest Nigeria has forced about 23,000 refugees to flee to Niger since April and raised concerns about the deteriorating security situation, the United Nations refugee agency said on Tuesday.

The numbers fleeing to neighbouring Niger have almost tripled from last year when the agency reported the first influx of 20,000 people following an insurgency and banditry in northern Nigeria which killed hundreds and displaced thousands.

The latest influx of mostly women and children came after attacks by gunmen in Nigeria's Katsina, Sokoto and Zamfara states during April.

The deadliest attack claimed 47 lives in Katsina State, the agency said, prompting air strikes by Nigerian security forces already stretched tackling a decade-long insurgency by Islamist group Boko Haram in the northeast.

"We are working closely with authorities in Niger to relocate at least 7,000 refugees to safety ... where water, food, shelter, access to health and other essential assistance can be provided," UNHCR spokesman Babar Baloch told a media briefing.

"Discussions are also ongoing with the authorities to recognize on a prima facie basis the refugees fleeing Nigeria and arriving in the region," he said.

Nigeria closed all land borders in March to curb the spread of the coronavirus pandemic, which has infected over 4,600 people in the country with 150 deaths. It first shut parts of its borders last year to fight smuggling but people could still cross both ways.

The agency said refugees from Nigeria are being allowed to seek protection in Niger despite border closures with people in need of food, shelter and basic services including healthcare.

Baloch said around 19,000 Niger nationals have been displaced in their own country as they fled, fearing insecurity in border areas. The refugees are found in Niger's southern Maradi region, the agency said.

Many have also been caught up in clashes blamed on farmers and herders over dwindling land in Nigeria which have killed more people than the Boko Haram conflict.