Simplice Kpandji, a U.N. spokeswoman, said she didn't know whether anyone at the Mugunga III camp had been killed, CNN reported.
The camp, which houses more than 30,000 refugees, is about 6 miles from Goma, the city that has been the center of fighting between the M23 rebels and government troops.
Kpandji said officials were working on gathering more information to identify the attackers.
U.N. monitors determined that rebels had withdrawn from Goma, more than a week after seizing the city from government forces, just as word came of the attack on the camp.
The United Nations and Goma residents accused the M23 rebels of looting, and witnesses said government troops also looted as they fled Goma last week.
The United Nations welcomed M23's move, but said the group must cease military activity and stay outside of Goma.
"We welcome [the M23s'] withdrawal, but emphasize that this is an early step and that stability in the region remains very fragile," Kieran Dwyer, a spokesman for U.N. peacekeepers, said.