Islamic State in Nigeria might kill health care workers it has held hostage since March within 24 hours, the International Committee of the Red Cross (ICRC) said on Sunday, pleading for mercy and urging Nigeria's government to intervene.

Medical workers Hauwa Mohammed Liman and Alice Loksha were working in the town of Rann when they were kidnapped in March along with ICRC midwife Saifura Hussaini Ahmed Khorsa, who was killed in September, the ICRC said in a statement.

The armed group was also holding a 15-year-old schoolgirl, Leah Sharibu, who was abducted in February from her school in the town of Dapchi, it said.