"We've seen the reports. We are reaching out to the de facto authorities in Niger to better understand what this means and the implications for the humanitarian work," U.N. spokeswoman Alessandra Vellucci told reporters in Geneva.
Her comments came after Niger's interior ministry late Thursday announced it was stopping U.N. agencies, NGOs and international organizations from working in military "operation zones."
It did not specify which regions were affected, but said the measures were "due to the current security situation."
"All activities and or movements in the zones of operations are temporarily suspended," it said.
Niger's new military leaders took power in a coup on July 26, when troops ousted President Mohamed Bazoum.
But since the coup, the attacks have continued, with the U.N. refugee agency saying earlier this week that more than 20,000 people had been displaced by such violence in the past month alone.
More than 710,000 people are already displaced within the country, including uprooted Nigeriens and refugees and asylum seekers from neighboring countries.
The U.N.'s International Organization for Migration warned Friday of overcrowding in the migrant transit centers it runs in Niger.
The organization hosts around 5,000 migrants in seven transit centers positioned along Niger's migration routes.
It said that more than 1,400 people — mainly from Mali, Guinea, Senegal and Nigeria — had been unable to access the overcrowded centers and were outside awaiting assistance.
IOM is calling for the establishment of humanitarian corridors to allow people to return to their countries of origin, and thus relieve the strain on the centers.
"At this time, there are no possibilities for us to organize charter flights, and consequently people will remain there for weeks and months," Christopher Gascon, IOM's regional director for West and Central Africa, told journalists in Geneva.
Opening up humanitarian corridors would allow aid workers to bring migrants to the airport and organize charter flights out, he said.