White House officials said Friday they cannot confirm the Islamic State group's claim that a Jordanian airstrike killed a female American aid worker held hostage by the group.
"We are obviously deeply concerned by these reports," said National Security Council spokeswoman Bernadette Meehan. "We have not at this time seen any evidence that corroborates ISIL's claim."
The family of aid worker Kayla Mueller, of Prescott, Arizona, also released a statement late Friday, saying they still were hopeful she is alive.
"This news leaves us concerned, yet we are still hopeful that Kayla is alive," Carl and Marsha Mueller said of their 26-year-old daughter. They asked her captors to "respond to us privately."
The Islamic State said Mueller was killed when a Jordanian airstrike hit the building where she was being held in the Syrian city of Raqqa. She had been captured when she left an Aleppo hospital she was visiting in 2013.
Jordan has been bombarding Islamic State targets since Thursday, following the release this week of a video showing the murder of a Jordanian fighter pilot who was captured in Syria in December. The slickly produced video shows pilot Muath al-Kaseasbeh locked in a cage, doused with fuel and burned alive.
The claim that she was killed in an airstrike was greeted with skepticism, after Jordanian authorities said their pilot was killed a month before IS released the video showing his death.
Jordanian officials call Islamic State's claim "criminal propaganda." Foreign Minister Nasser Judeh said on Twitter it is "an old and sick trick used by terrorists and despots for decades – claiming that hostage human shields held captive are killed by air raids."
U.S. State Department spokeswoman Marie Harf said the State Department has acknowledged there are Americans being held overseas, including by the Islamic State, but said she could not get into any further details about this specific case.
Those who know Kayla say she was deeply moved by the suffering of civilians stuck in poverty and caught up in war and terrorism. She had worked in Syria and volunteered with aid groups in the West Bank and India.
On Friday in Jordan's capital, Amman, thousands of people flooded the streets in support of the murdered pilot.
The demonstrators marched through the city after Friday prayers at the al-Husseini mosque, waving Jordanian flags and chanting slogans against the Islamic State group.
Jordan said recent airstrikes against Islamic State targets are only the beginning of its retaliation.
"The Jordan army's response to ISIS [Islamic State militants] and what they have done to Muath and all of the other innocent people is totally justified," said Ayman Khamoudeh, a doctor taking part in Friday's protests. "If no one puts a struggle against them and fights them, who else will fight them?"
Jordanian warplanes launched dozens of airstrikes against the Islamic State group in Syria and Iraq Thursday after King Abdullah vowed an "earth-shaking" response to the killing.
Abdullah visited Kaseasbeh's family at their home Thursday. He said Jordan will continue the fight to defend its people and religion.
Judeh, the foreign minister, told CNN television late Thursday that Jordan will go after Islamic State militants "wherever they are, with everything we have."
"We're going to go after them and we will eradicate them," said Judeh, who referred to Islamic State fighters as "cowards."
U.S. Defense Department officials said Thursday the Pentagon has moved aircraft and pilots to northern Iraq to better position themselves in search and rescue missions in Iraq and Syria. Some of the assets were moved from Kuwait.
Also Thursday, a leader of Kurdish peshmerga fighters told VOA his ground forces are in need of military assistance in their fight against Islamic State
"NATO and the United States should understand that we need political and military assistance," said commander Wasta Rasul. "I ask formally that their ground forces participate in the war with us. And why should the Kurds do it alone as we fight on behalf of the world?"
Material for this report came from AP, AFP, and Reuters.