Earnest said officials believed that the "Islamic State" (IS) held one US citizen. News agencies speculated it could be journalist Austin Tice, who disappeared in Damascus in 2012.

"We have avoided discussing the individual cases of Americans who have been held hostage, but we are aware of other American hostages being held in the region," Earnest said on Tuesday in response to a question about whether IS or similar groups held any other US citizens hostage.

The news came after the death of an American in IS custody and as the White House has sought to push a widely anticipated request by President Barack Obama for legislation approving the use of US military force against the group. On Tuesday, White House chief of staff Denis McDonough and counsel Neil Eggleston met with Senate Democrats as Obama prepared to formally unveil his proposed authorization.

Obama announced that Kayla Jean Mueller, an aid worker captured while leaving a Doctors Without Borders center in Aleppo and held hostage by IS, died during her imprisonment. "No matter how long it takes, the United States will find and bring to justice the terrorists who are responsible for Kayla's captivity and death," Obama said in a statement released Tuesday.

Last week, IS claimed that Mueller died during a series of bombings conducted by the Jordanian military to avenge a fighter pilot's death. IS had posted a video of the man being burned alive in a cage online as part of its viral campaign to instill fear and attract support.

'We are heartbroken'

During her captivity, Mueller, 26, took strength in her faith and her family's love, she wrote in a letter released on Tuesday. The humanitarian aid worker had an unquenchable passion to help others and said that service brought her closer to God.

"I have been shown in darkness, light + have learned that even in prison, one can be free," the Arizonan wrote in the letter, smuggled out by captives and received by her relatives in early 2014. "I have come to see that there is good in every situation, sometimes we just have to look for it," she added.

Mueller described her prison as "a safe location" and said she was "completely unharmed + healthy (put on weight in fact)." She revealed doubts, but also resolve as she summoned the will to keep going.

"We are heartbroken to share that we've received confirmation that Kayla Jean Mueller has lost her life," said her parents, Carl and Marsha. "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."

mkg/bw (Reuters, AFP, dpa, AP)