Islamic State militants are holding a third American hostage - a woman who worked for a humanitarian organization - and are demanding a ransom of US$6.6 million for her freedom.

ABC News reports that Aside from the ransom money, the IS is also demanding that the United States release Dr. Aafia Siddiqui, a Pakistani neuroscientist who trained at MIT. Siddiqui was convicted and sentenced to 86 years imprisonment in 2010 for trying to kill U.S. officials in 2008. She is now detained at the Federal Medical Center in Fort Worth, Texas.

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The family of the female hostage has requested that she not be identified. But other sources said the hostage is 26 years old, and was serving with an international humanitarian group when she was abducted over a year ago. The woman is said to be from the West Coast.

Earlier, the IS had also demanded Siddiqui's  release as a condition for the James Foley's freedom. Foley was executed last week as shown in a video circulated by the IS over the internet.

The U.S. State Department would not confirm whether there was an American woman being held by IS. One other American, Steven Sotloff, was shown in the James Foley video as being an IS prisoner. Militants have threatened to execute Sotloff unless the U.S. stops airstrikes against their positions in Syria and Iraq.

On Monday, the family of Dr. Siddiqui spoke out through supporters and said they were distraught at Siddiqui's name being invoked by the Islamic State.

A letter written by Siddiqui's family and read in a press conference denied any connection with the terrorist organization.

"We would like to state that our family does not have any connections to such groups or actions. We believe in a struggle that is peaceful and dignified. Associating Aafia's name with acts of violence is against everything we are struggling for," the family said in the letter.

Mauri Saalakhan of the Peace and Justice Foundation, who was at Monday's press conference, said the family did not want "harm to come to anyone else's loved one in the name of Aafia."

Saalakhan's group have been involved in a lengthy campaign to secure Siddiqui's freedom, believing that the latter's imprisonment was unjust.

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