Boko Haram militants have killed a female aid worker held by the group since March, the Nigerian government has confirmed.
In September, the jihadist group killed a midwife taken at the same time.
The militants have also been holding a 15-year-old schoolgirl and another medical worker.
On Sunday, the International Committee of the Red Cross (ICRC) appealed for their lives to be spared as a deadline issued by the militants approached.
Nigerian Information and Culture Minister Alhaji Lai Mohammed said on Monday the government was “deeply pained” by news of the killing, but added it would “keep the negotiations open and continue to work to free the innocent women who remain in the custody of their abductors”.
Medical workers Hauwa Mohammed Liman and Alice Loksha were kidnapped alongside midwife Saifura Hussaini Ahmed Khorsa in March while working in the town of Rann with the ICRC.
A schoolgirl who is also being held was among 110 girls kidnapped from a school in the town of Dapchi in February. Most of the other students were freed but the girl, who refused to convert to Islam, remains in captivity.
Ms Khorsa was killed last month and the faction holding her – known as the Islamic State’s West Africa Province – threatened to kill another hostage by Monday if their demands were not met.
An ICRC spokeswoman told Reuters on Monday: “We are hearing devastating reports that Hauwa has been executed. At this stage, we have no confirmation that this is true.
We desperately hope not. This situation is heartbreaking and our thoughts remain with the family.”