Thousands of civilians have fled on foot after Boko Haram forces overran the Niger town of Bosso. Niger troops were battling the jihadists for the control of the border settlement.
The majority of the displaced people were trying to reach the nearby town of Toumour, some 30 kilometers (18 miles) west of Bosso, UNHCR said.
"Many people are reportedly traumatized and worried about their safety," spokesman Adrian Edwards told reporters in Geneva. "People are sleeping in the open and urgently need shelter and other assistance."
Bosso residents started fleeing on Saturday after a deadly Boko Haram attack, a local journalist told the AFP news agency. The jihadi group allegedly used heavy artillery to target the town's garrison on Friday evening, he said.
"They came in large numbers shouting 'Allahu Akbar' (God is Great)," the reporter said.
The source, who refused to be named, added that the militants were "burning the military barracks, police facilities and local administration office before looting shops and carting away food supplies."
"Everybody started leaving the town and surrounding villages at dawn after the gunmen had left," he added.
Around 30 Niger soldier were killed during the battle, alongside two troopers from neighboring Nigeria. Also, 55 insurgents died in the attack, according to the authorities.
However, the militants returned on Sunday and attacked the city once again. There were conflicting reports about the battle on Tuesday.
Millions with no home
"The welfare of these people and others forced to flee the violence in Bosso is of great concern," UNHCR spokesman Edwards said.
"A team will be en route to the Diffa region this week," he added.
Last month, Niger government estimated that the clashes had displaced over 240,000 people in the area. Overall, the Boko Haram insurgency has killed tens of thousands across Nigeria, Niger, Chad, and Cameroon over the past seven years.
At least 2.6 million people have been forced to flee their homes.