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."
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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.