The Islamic State militant group is holding hostage a young American woman who was doing humanitarian aid work in Syria, a family representative said Tuesday. The 26-year-old woman is the third American known to have been kidnapped by the militant group.

The Islamic State group recently threatened to kill American hostages to avenge the crushing airstrikes in Iraq against militants advancing on Mount Sinjar and the Kurdish capital of Erbil.

The 26-year-old woman was captured last year while working with three humanitarian groups in Syria. A representative for the family and U.S. officials asked that the woman not be identified out of fear for her safety. All spoke on condition of anonymity because they were not authorized to discuss the issue publicly.

Journalist James Foley was honoured Sunday at a Mass in Rochester, N.H. He was kidnapped and killed by ISIS militants while covering the war in Syria. (Nicole Tung/freejamesfoley.org/Associated Press)

A ransom demand of $6.6 million US has been made, ABC News reported.

More than a week ago, freelance journalist James Foley of Rochester, N.H., was beheaded by the Islamic State group, which kidnapped him in November 2012. Foley, 40, has worked in a number of conflict zones across the Mideast, including Iraq, Libya and Syria. He was in northern Syria on assignment for Agence France-Press and the Boston-based media company GlobalPost when the car he was riding in was stopped by four militants in a contested battle zone that both Sunni rebel fighters and government forces were trying to control.