Niamey Nearly 7,000 people have fled the region in western Niger where jihadist fighters killed 89 soldiers in a devastating attack earlier this month, the UN refugee agency said.
But insecurity in the region is making it difficult for the UNHCR to bring them the help they need, the statement added.
Following the attack at the town of Chinegodar, around 20 kilometres (12 miles) from the Malian border, "an estimated 7,000 civilians including 1,000 refugees have been forced to flee their homes in search of safety," the statement said.
"We are receiving accounts of people being given an ultimatum by armed groups to leave the area, of civilians being targeted, kidnapped or killed, of properties being looted."
Those who fled had to leave with little more than the clothes on their backs, UNHCR said.
"They are in urgent need of food, water and shelter, as well as sanitation, protection and security," the statement added.
"Insecurity in these areas severely hampers our ability to reach the affected population, those forced to flee and the communities hosting them," it said.
"In Sahel, the protection of those forced to flee must be at the core of the response to this displacement crisis."
Last week the Islamic State group claimed responsibility for the January 9 attack on the army base, one their deadliest in the region.
While the official death toll was 89, the jihadists claimed they killed more than 100 people. The authorities in Niger say their troops killed 77 of the attackers.
In Niger, some 5,000 people fled to the nearby towns of Banibangou or Oualam, which are already hosting more than 7,000 Malian refugees, UNHCR said.
Around 1,000 Nigerien refugees, including unaccompanied children, have crossed over into Mali at the border town of Anderaboukane in the Menaka region, receiving help from local people.
According to UN figures, jihadist fighters killed more than 4,000 people across the Sahel region in 2019.
There is growing concern from Libya's neighbours including Niger that they are suffering the fallout from the increasing instability there.
African Union Chairman Moussa Faki is among those attending Sunday's Berlin talks on Libya as African countries seek a greater say in the peace process.