NICK CUMMING-BRUCE

GENEVA — As international support builds for the search for more than 200 abducted Nigerian schoolgirls, the United Nations refugee agency expressed alarm Friday at the swelling tide of people fleeing their homes in northeast Nigeria to escape attacks by Boko Haram insurgents that it said were unprecedented in their brutality and frequency.

Up to a thousand people are crossing the border into southern Niger every week from fear of attacks by the Islamist insurgent group and counterattacks by the Nigerian armed forces, and smaller numbers have arrived in Cameroon and Chad, Adrian Edwards, a spokesman for the refugee agency, told reporters.

Refugees are providing accounts of grenade attacks on markets killing villagers and livestock, summary executions and whole villages and their crops being burned to the ground.

“Some have witnessed friends or family members being randomly singled out and killed in streets,” Mr. Edwards said, while others had spoken of arbitrary arrests on suspicion of belonging to insurgent groups.

Almost one year after Nigeria declared a state of emergency in three northeastern states, the Nigerian authorities have reported that a quarter of a million people are now displaced within the country and more than 60,000 people have fled across borders, Mr. Edwards added.

American and British security experts arrived in Nigeria this week to help locate more than 200 missing schoolgirls seized by Boko Haram militants from their boarding school in the town of Chibok in the northeastern state of Borno on April 14. It is believed that the girls are being held somewhere in the forests that stretch into Cameroon. France also offered this week to send an intelligence team to support the search.

But that attack was only one in a series of kidnappings from schools in northeastern Nigeria in recent months, the refugee agency noted, citing reports from students who survived such attacks and described friends being killed or kidnapped.

Some 1,500 people had arrived in one village in southern Niger as a result of an attack on April 20 involving six insurgents who burned houses, shops and food stocks, Mr. Edwards said.

After crossing the border, those fleeing violence were still at risk because of a lack of security and the remoteness of the region, Mr. Edwards said, noting the refugee agency had moved people arriving in northern Cameroon from Borno to a location 25 miles from the border for their safety.