The victim was abducted during a raid on a military facility in the area on March 1.
At least four soldiers, policemen and three humanitarian workers were killed in the attack.
Hauwa Leman, one of the workers abducted, had sent an audio to one of her friends, demanding that her parents be informed that she had been kidnapped.
Following the murder of Ahmed, Leman and Alice Ngaddah, a mother of two, are the ones in captivity among the three people abducted in Rann on March 1.
In a video seen by TheCable, the deceased was wearing a white hijab. She was shot from behind.
The insurgents claimed that they had contacted government over the captives but did not get any response.
“We contacted the government through writing and also sent audio messages but the government have ignored us. So, here is a message of blood,” said a spokesman of the group who did not give his name.
“The other nurse and midwife will be executed in similar manner in one month, including Leah Sharibu.”
The ICRC has confirmed the identity of the victim, promising to issue a statement.
In August, TheCable obtained an audio of Sharibu appealing to President Muhammadu Buhari to rescue her.
ISWAP has kept her in captivity for seven months, sparking public outrage.
The federal government had said it was making efforts to secure her release.
Sharibu is the only Christian among the abducted 119 students of Government Girls Secondary School, Dapchi, Yobe state, kidnapped in February.
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