President Barack Obama has led the tributes to an American woman killed while being held hostage by Islamic State in Syria, saying she represents what is best about America.

Islamic State militants have been blamed for the death of aid worker Kayla Mueller, who IS claimed was killed in a Jordanian air strike.

Ms Mueller's family received an email and photograph at the weekend from her captors that led US intelligence to determine she had been killed, officials said.

President Obama vowed to bring to justice her killers "no matter how long it takes".

"ISIL is a hateful and abhorrent terrorist group whose actions stand in stark contrast to the spirit of people like Kayla," he said.

"On this day, we take comfort in the fact that the future belongs not to those who destroy, but rather to the irrepressible force of human goodness that Kayla Mueller shall forever represent."

Ms Mueller was captured in August 2013 while leaving a hospital in Aleppo in northern Syria.

Her family said in a statement: "Kayla was a compassionate and devoted humanitarian.

"She dedicated the whole of her young life to helping those in need of freedom, justice, and peace."

They have also released a letter she wrote while in captivity.

In it, she stated she was "in a safe location, completely unharmed + healthy" and had been treated with "the utmost respect + kindness".

"Just the thought of you all sends me into a fit of tears," she wrote.

IS said on Friday that the 26-year-old, from Prescott, Arizona, was killed when Jordanian fighter jets bombed a building outside Raqqa where she was being held.

But both Jordanian and US officials have cast doubt on that account.

On Tuesday, White House spokesman Josh Earnest referenced the air strike, which was carried out on a weapons compound in conjunction with US support.

"The information that we have is that there is no evidence of civilians in the target area prior to the coalition strike taking place," he said.

Pentagon spokesman Navy Rear Admiral John Kirby said US officials are certain Ms Mueller did not die in any of the air strikes launched by Jordan in retaliation for the killing of one of its pilots.

Two US national security officials who have closely followed the situation said it appeared most likely she was killed in some sort of combat situation in which her captors were unable to keep her safe.