WASHINGTON — 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 Irbil.

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 the woman not be identified out of fear for her safety.

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.