Either of two aid workers kidnapped by a faction of the Boko Haram terrorist group in Nigeria may have only hours to be rescued before being executed, according to a fervent appeal by the aid organizations they work for.

Hauwa Muhammed Liman, a 24-year-old midwife at a government health care center supported by the International Committee of the Red Cross, and Alice Loksha, a nurse working for the United Nations agency Unicef, were abducted along with another aid worker in March, when militants thought to belong to a Boko Haram faction stormed Rann, a northeastern Nigerian town where there are tens of thousands of refugees.

The faction, the Islamic State West African Province, which is supported by ISIS, has been responsible for high-profile abductions and propaganda victories, further complicating the security crisis from an insurgency entering its 10th year.

On Sept. 16, another aid worker, Saifura Khorsa, a 25-year-old nurse with the international Red Cross group, was executed by the faction. Ms. Liman and Ms. Loksha remain captives, as does Leah Sharibu, a Christian student who refused to convert to Islam and was seized in February.