President Barack Obama on Tuesday confirmed the death of US hostage Kayla Mueller, as warplanes from Gulf ally the United Arab Emirates rejoined the battle against her captors from the Islamic State group.

Obama vowed that Washington would hunt down the jihadists it held responsible for the 26-year-old aid worker's death, as her family spoke of their heartbreak.

"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.

Mueller was captured in August 2013 in Aleppo, the war-ravaged city in northern Syria.

IS claimed last week she had been killed in an air strike by a Jordanian warplane in Raqa, the militant group's self-proclaimed "capital," also in Syria.

US officials on Tuesday said they remained skeptical of IS claims Mueller died in an air strike, noting there had been no evidence of civilians at that site before it was targeted.

"That certainly would call into question the claims that are made by ISIL," White House spokesman Josh Earnest said, using an alternate acronym for IS, which has grabbed large areas of Iraq and Syria in a brutal offensive of beheadings and forced religious conversions.

The White House said IS had sent Mueller's family a "private message" that was "authenticated" by intelligence, allowing them to confirm her death. The Washington Post reported that Mueller's parents had been sent a photo of their daughter's body.

"Our hearts are breaking for our only daughter, but we will continue on in peace, dignity, and love for her," her parents Carl and Marsha Mueller said.

In a letter from captivity in early 2014, Mueller said she was "completely unharmed" by her captors but heavy-hearted about the toll it must have taken on her family.

"Just the thought of you all sends me into a fit of tears," she wrote in the letter released by her family.

– Pilot's gruesome death –

Early Tuesday, warplanes from the UAE launched anti-IS air strikes, as the important Arab ally in the US-led coalition against the IS extremists resumed combat operations.

The F-16s hit their targets and returned safely to base, the UAE military said, without saying what their targets were.

Following the December crash and capture in Syria of Jordanian F-16 pilot Maaz al-Kassasbeh, the UAE withdrew from coalition strike missions, fearing for the safety of its pilots.

The jihadists later killed Kassasbeh by burning him alive, releasing gruesome video footage of his "execution."

The coalition began air strikes against IS in Syria on September 23, but has pointedly refused to coordinate with Damascus.

On Tuesday, the Pentagon said coalition aircraft had launched one strike in eastern Syria in the 24 hours to 0600 GMT, and also pounded the jihadists in Iraq with 11 strikes.

– 'There is no dialogue' –

Syria has grudgingly accepted the air strikes against IS, but has repeatedly criticized the coalition for failing to coordinate with it.

It says the raids cannot defeat IS unless the international community starts cooperating with Syrian troops on the ground.

In an interview broadcast by the BBC, Syrian President Bashar al-Assad confirmed there was no cooperation with the coalition, members of which he accused of backing "terrorism" — an apparent reference to their support for other rebel groups fighting to overthrow him.

"Sometimes, they convey a message, a general message. There is no dialogue. There's, let's say, information, but not dialogue," Assad said.

Assad said the US-led strikes had the potential to help his government, but that so far they were not sufficiently "serious" to do so.

The Syrian regime has been accused of indiscriminate bombardment of civilians in rebel-held areas, including with barrel bombs — crude munitions packed with explosives and shrapnel.

But Assad flatly denied the allegation as a "childish story."

"I haven't heard of the army using barrels, or maybe cooking pots," he said, laughing.

He also denied claims that government forces had used chemical weapons in an August 2013 attack outside Damascus that killed up to 1,400 people.

"Who verified who threw that gas on who?" he asked.

Challenged on whether his government did so, he said "definitely not," adding that the reported death toll was "exaggerated."

Since Syria gave up its chemical arsenal after the 2013 attack, there have been persistent reports of the use of chlorine gas.

Assad said his forces were "definitely not" using chlorine as a weapon.

More than 210,000 people have been killed in Syria since the conflict began with anti-government protests in March 2011.

American Kayla Mueller: A calling to ease others' suffering
Washington (AFP) Feb 10, 2015 –

Kayla Mueller, who died as a hostage of IS jihadists, took strength during captivity in her faith in God and the love of her family, she wrote in a letter relatives released on Tuesday.

"I have been shown in darkness, light + have learned that even in prison, one can be free," said a handwritten letter by Mueller which was smuggled out by fellow captives following their release by Islamic State jihadists.

The 26-year old humanitarian aid worker was driven by an unquenchable passion to help others, saying that service to others brought her closer to God.

In the letter released by her family after the White House confirmed her death, she said she managed to find glimmers of joy, even in captivity.

"I have come to see that there is good in every situation, sometimes we just have to look for it, Mueller wrote in the letter received by her relatives in early 2014.

Mueller described the place where she was being held as "a safe location," and said she was "completely unharmed + healthy (put on weight in fact)."

She also revealed times of doubt and darkness, but also resolve as she summoned the will to keep going.

"None of us could have known it would be this long but know I am also fighting from my side in the ways I am able + I have a lot of fight left inside of me," she wrote in her letter.

– Humanitarian service –

The idealistic native of Prescott, Arizona, was captured by IS fighters in August 2013, while leaving a Doctors Without Borders hospital in Aleppo, Syria.

Her humanitarian service over the years included volunteer work at a women's shelter in her hometown of Prescott, as well as work at an HIV/AIDS clinic.

At the time she was taken hostage, Mueller had been working along the Turkish-Syrian border, trying to help some of the thousands of refugees fleeing the Syrian civil war.

That conflict was but one of several where Mueller had hoped to make a difference.

Since her graduation from Northern Arizona University in 2009, she dedicated her life to helping those in need across the globe from India, to Israel, to the Palestinian territories.

Mueller was the last remaining American hostage known to be held by the jihadist group, which has made a grisly ritual of killing a succession of hostages, then posting the brutal executions on the Internet.

Mueller's relatives only last week revealed that she was in fact, being held by IS, after having requested to keep her captivity secret since her abduction.

After IS last week announced that Mueller had died as a casualty of coalition bombing, her family continued to cling to the hope that she somehow had escaped death.

But over the weekend they received a "private message" from IS that allowed US intelligence services confirm her death, the White House and the family said.

– Life with purpose –

The family said they are buoyed by the knowledge of how much good Mueller did during her short life.

"She lived with purpose, and we will work every day to honor her legacy," they said.

Eryn Street, a friend, later told reporters in her hometown of Prescott: "I'm not yet sure how to live in a world without Kayla, but I do know that we're all living in a better world because of her."

One of her aunts, Lori Lyon, read out a statement from her aunts and uncles, saying: "She had a quiet, calm presence. She was a free spirit, always standing up for those who were suffering and wanting to be their voice.

"Kayla's calling was to help those who were suffering, whether in her hometown of Prescott, or on the other side of the world," Lyon added.

President Barack Obama paid homage to Mueller's brief, purpose-driven existence.

"She epitomized all that is good in our world," the president said in a statement.