The attack on Damasak “killed up to 50 people and forced at least 3,000 to flee for their lives to Diffa region in neighbouring Niger,” said Adrian Edwards, spokesman for the UN refugee agency.
Boko Haram, which wants to carve out a strict Islamic state in northern Nigeria, took over the town in the far north of Borno state on Monday, starting their assault with an attack on a crowded market there.
“Our teams in Diffa say that people are still arriving in Niger from Nigeria as a result of the attack,” Edwards told reporters in Geneva.
He said many were crossing in boats, but also said locals had described heartbreaking scenes of people drowning as they tried to swim for safety.
“Others were reportedly shot by Boko Haram who chased them as far as the river banks,” he said.
People arriving in Niger were telling UNHCR workers there that many people, mostly women, children, elderly and the injured, were still waiting on the Nigerian side to cross the river.
“The new arrivals are telling us that many civilians were killed during the attack on Damasak, especially young men, but the insurgents were also shooting at women and children,” Edwards said.
There were reports that the attack had been carried out in reprisal for young men in the town enrolling in self-defence groups formed to fight the insurgents, he said.
The attack had come suddenly, and most people had fled the town with nothing, Edwards said.
“Many children have been separated from their parents during the attack and the escape to Niger,” he said, describing how children and adults were seen wandering around the closest town, Chetimari, searching for relatives.
The attack on Damasak came after a similar raid last weekend that left 48 people dead in the nearby town of Doron Baga.
Edwards also pointed to the attack on the village of Malan Fatori last month that prompted more than 1,000 people to flee to Niger.
More than 100,000 people have fled Nigeria for Niger since May 2013, according to UNHCR, which estimates that more than 30,000 of them have fled in the past two months.
And “the increasing presence of the insurgents in close proximity to the border with Niger could lead to new displacements in the near future,” Edwards said, warning that the “already fragile economic structure” in the impoverished region “could collapse under the strain”.
In addition to those who have fled to Niger, 39,000 Nigerians have fled to Cameroon and 2,800 have fled to Chad.
The Boko Haram insurgency, which is estimated to have cost more than 13,000 lives since 2009, has also displaced some 700,000 across northeastern Nigeria, according to government numbers.