The ICRC issued desperate pleas to the Nigerian government to negotiate the release of the workers, adding that, "speed and urgency are critical." Aleksandra Matijevic Mosimann, ICRC spokeswoman in Nigeria, told CNN they received an ultimatum from the militants on September 16 when the group also issued a threat against schoolgirl Leah Sharibu, whose parents confirmed to CNN she was "facing a death sentence." Mosimann said the ICRC does not negotiate with terrorists but Boko Haram had reached out to them through "credible sources," about the deadline. "We don't know the exact hour, but we have information from credible sources about this ultimatum after they killed one of our aid workers last month. That is why we have issued a public plea to the Nigerian government for their release," Mosimann said.Saifura Hussaini Khorsa, 25, was kidnapped in March this year Hauwa Mohammed Liman and Alice Loksha during an attack on a military facility in Rann, northeast Nigeria. Khorsa was executed last month and since then the clock has been ticking on the fate of the other two aid workers. ICRC urged those negotiating their release to do "everything they can to avoid a repeat of that devastating outcome."We urge you: spare and release these women. They are a midwife, a nurse, and a student. Like all those abducted, they are not part of any fight," said Patricia Danzi, Director of ICRC Operations in Africa."They are daughters and sisters, one is a mother — women with their futures ahead of them, children to raise, and families to return to," she added.International pressure continues to mount on the Nigerian government following the ultimatum issued by the insurgents.Nigeria's President Muhammadu Buhari said his administration was doing all it can to secure their release, including working with international negotiators, who helped to secure the release of the kidnapped Chibok schoolgirls.