BEIRUT (AP) - A statement attributed to Islamic State extremists claimed that an American woman held hostage by the group was killed on Friday, allegedly in a Jordanian airstrike on the militants' stronghold of Raqqa in northern Syria.

The woman was identified as Kayla Jean Mueller, an American who went to Syria to do aid work, but there was no independent verification of the claim and the United States said it had no evidence of her death.

The 26-year-old Mueller, of Prescott, Arizona, is the only known remaining U.S. hostage held by the Islamic State group. Journalist Austin Tice, of Houston, Texas, disappeared in August 2012 while covering Syria's civil war. It's not clear what entity is holding him, but it is not believed to be the Islamic State group or the Syrian government, his family has said.

If Mueller's death is confirmed, she would be the fourth American to die while in the captivity of the Islamic State militants. Three other Americans, journalists James Foley and Steven Sotloff and aid worker Peter Kassig were beheaded by the group.