Three aid workers have been released after being kidnapped by armed militants in Borno state on 22 December.
The UN says the humanitarians were released on Wednesday with a number of civilians.
It is unclear whether a ransom was paid, or who carried out the abductions, but the militant group Boko Haram has been active in Borno for more than a decade.
A representative from the UN’s Office for the Coordination of Humanitarian Affairs (Ocha) said he remained concerned about the fate of aid worker Grace Taku, abducted in July 2019, and Alice Loksha, a nurse kidnapped almost two years ago. Both women remain in captivity.
Edward Kallon added in his statement that an increase in unofficial checkpoints by “non-state armed groups” in the north-east was putting civilians and aid workers at risk.
According to the UN, 12 humanitarians died in Nigeria in 2019, double that of the previous year.