The slain aid workers all were men working in Rann in far northeast Nigeria, according to a statement Friday by the UN's Office for the Coordination of Humanitarian Assistance
. Two were working for the International Organization of Migration, and one was a contracted medical doctor working for UNICEF.
A woman working as a nurse was missing after the attack and is feared to have been kidnapped, UNOCHA said.
The United Nations also was "concerned" about other civilians who may have been killed or injured in the attack, the agency said.
The attackers were not immediately identified.
Rann, a remote town outside Maiduguri, the capital of Borno State, hosts a camp of about 55,000 internally displaced people.
Boko Haram has regularly attacked such camps with gunmen and suicide bombers.
About 3,000 aid workers, most of them Nigerian nationals, work in northeast Nigeria.
"Aid workers put their lives on the line every single day to provide emergency assistance to vulnerable women, children and men," said Edward Kallon, UN's humanitarian coordinator in Nigeria.
"Our deepest condolences go to the families of the victims and our brave colleagues, and we call on authorities to ensure the perpetrators are brought to justice and account," he added.