WASHINGTON (AFP) - Sombre US lawmakers stood with heads bowed Wednesday during a moment of silence for American aid worker Kayla Mueller, who was held hostage by Islamic State in Iraq and Syria (ISIS) extremists and died.

The House of Representatives' nine-member delegation from Arizona, Ms Mueller's home state, led colleagues in the 12-second silent tribute to a 26-year-old woman that Republican Paul Gosar described as "one of our own."

Ms Mueller, who was seized in the Syrian city of Aleppo in August 2013, "stood as a beacon of light and hope" during a period of intense turmoil and unrest in the Middle East and elsewhere, the congressman said.

"Let us look back fondly upon her life," he said.

President Barack Obama and Ms Mueller's family on Tuesday confirmed her death, with Mr Obama pledging that Washington will hunt down the radicals it held responsible.

Earlier, ISIS extremists claimed Ms Mueller was killed in a Feb 6 coalition air strike that buried her in rubble.

United States officials pushed back on the claim and have said the circumstances of Ms Mueller's death remain unclear.

In the opening paragraph of Mr Obama's letter to Congress seeking war authority against ISIS, Mr Obama said ISIS was "responsible" for the death of Ms Mueller and three other Americans who had been held captive by the group.