Hauwa Mohammed Liman, who worked in Red Cross-supported hospital, had been held hostage since March
Islamist extremists in Nigeria have killed a medical aid worker held hostage since March.
Hauwa Mohammed Liman, 24, was killed by militants from a faction of Boko Haram after a deadline expired, authorities have said.
Liman, a Nigerian who worked in a hospital supported by the International Committee of the Red Cross (ICRC), was one of three aid workers kidnapped by extremists during a raid on the town of Rann in the restive north-eastern Borno state.
A second aid worker, an ICRC midwife, was killed in September. The surviving hostage worked as a nurse in a centre supported by Unicef.
Boko Haram, which has also been known as Islamic State in West Africa, has been waging a deadly campaign in north-eastern Nigeria for almost a decade.
The militants said in a video posted online last month that they would kill at least one hostage once a deadline due to elapse on Monday had passed. It is unclear what demands, if any, the extremists made for the release of the hostages.
In a video released in recent hours viewed by local reporters, the militants said Liman deserved to be killed because she had abandoned Islam by working for the ICRC.
The clip had also referred to Leah Sharibu, a 15-year-old who was one of more than 100 schoolgirls abducted by Boko Haram from a boarding school in Dapchi in February.
While other students were released weeks after the abduction, Sharibu, the only Christian among them, was held back for refusing to convert Islam. The militants said the teenager would be kept as a slave.
Nigerias military and government have repeatedly said they are on the point of defeating Boko Haram and its various factions. However, raids on military bases have continued, inflicting significant casualties. The death toll from one assault on a Nigerian army post on the border with Niger last month reached 48. A similar attack was foiled this weekend, officials said.
The mass abduction of more than 200 schoolgirls from Chibok, Borno state, in April 2014 brought global attention to the insurgency and was widely condemned. More than 100 girls have since been released or found.
Lai Mohammed, the Nigerian information minister, said the government was deeply pained by the latest killing but pledged to keep the negotiations open and continue to work to free the innocent women who remain in the custody of their abductors.
The presidency tweeted that the federal government did all within its powers to save her life.
Analysts say Boko Haram is a fragmented coalition comprising different factions, rather than a single organisation. One major rift saw the group blamed for the recent deaths split from that led by Boko Harams veteran leader, Abubakar Shekau, after arguments over his indiscriminate targeting of civilians in raids and suicide bombings.
Analysts believe this breakaway faction has a new hardline leadership after another internal struggle and is responsible for the recent killings.