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BEIRUT -- The mother of a hostage American journalist pleaded for his release Wednesday in a video directed at the Islamic State group, while new images emerged of mass killings, including masked militants shooting kneeling men after the capture of a strategic air base in Syria.
Shirley Sotloff's plea came as a U.N. commission accused the group, which dominates a broad swath of territory spanning the Syria-Iraq border, of committing crimes against humanity and President Barack Obama weighs options for targeting the extremists' stronghold in Syria.
The Islamic State militants have threatened to kill 31-year-old Steven Sotloff unless the U.S. halts its airstrikes against it.
Sotloff, who freelanced for Time and Foreign Policy magazines, had last been seen in Syria in August 2013 until he appeared in a video released online last week by the Islamic State group showing the beheading of fellow American journalist, James Foley.
Dressed in an orange jumpsuit against the backdrop of an arid Syrian landscape, Sotloff was threatened with death unless the U.S. stopped airstrikes on the group in Iraq.
Addressing the leader of the Islamic State group by name, Shirley Sotloff said her son was "an innocent journalist" who shouldn't pay for U.S. government actions in the Middle East over which he has no control.
Appealing directly to Islamic State leader Abu Bakr al-Baghdadi, who describes himself as a caliph, or Islamic leader intending to lead the Muslim world, she implored him to show mercy and follow the example of the prophet Muhammad.
"You, the caliph, can grant amnesty. I ask you, please, to release my child. I ask you to use your authority to spare his life," Shirley Sotloff said on the video, which was first aired on the Al-Arabiya television network.
It was widely retweeted by Islamic State supporters later Wednesday with her face blurred because their ultra-conservative interpretation of Islam prohibits showing a woman's face.
At the White House, spokesman Josh Earnest said he did not know whether Obama had seen Shirley Sotloff's video appeal, but he said the administration was "deeply engaged" in trying to gain release of all Americans held hostage in the Middle East.
"She obviously, as is evident from the video, feels desperate about the safety and well-being of her son, and understandably so, and that is why our thoughts and prayers are with Mr. Sotloff's family at this very difficult and trying time," Earnest said.
Meanwhile, new images emerged of the extremists' bloody takeover of an air base in northeastern Syria.
In one photo posted on the militant group's website, masked gunmen were seen shooting seven men kneeling on the ground, some dressed in what appeared to be Syrian military uniforms, after the seizure of the Tabqa air base in the province of Raqqa earlier this week.
The photos underscored how the group uses violence, and images of violence to terrorize its opponents, as it sweeps further into Syria and Iraq, where it has imposed an Islamic state, or caliphate, governed by its harsh interpretation of Islamic law.
Some photos showed captured Syrian soldiers, many with bloodied and swollen faces. In one, a masked Islamic State fighter stood behind a group of soldiers brandishing a knife of the type the militants have used is the past to behead victims, including Foley. In another, a militant grinned as he pressed a double-edged sword against the neck of a captured soldier inside a jeep.
One photograph showed a headless corpse, while another showed 10 slain men, sprawled in a pool of blood on a dirt road. It wasn't clear if they were killed during the clashes or after being captured.
Videos uploaded to social media networks also showed the aftermath of the battle, including footage of the charred bodies of Syrian soldiers. One video showed about 200 captured soldiers being marched through the desert in their underwear to an unknown fate as militants made the sounds of shepherds herding goats or sheep.
The 36 photos and video images corresponded to reporting by The Associated Press of the Islamic State militants' seizure Sunday of the air base, which had been the last government-held outpost in Raqqa, a province now dominated by the jihadi group. Militants also captured the bases' weaponry, including artillery and mounted machine guns.
The images emerged as a U.N. commission accused the group of committing crimes against humanity in Syria. The U.N. had earlier accused the group of similar crimes in Iraq.
"This is a continuation -- and a geographic expansion -- of the widespread and systematic attack on the civilian population" by the Islamic State group, said the four-member commission chaired by Brazilian diplomat Paulo Sergio Pinheiro.
Pinheiro told reporters one of the most disturbing findings was the existence of large training camps where boys, some as young as 14, are recruited and trained to fight alongside adult Islamic State fighters.
The report, based on 480 interviews and documentary material, cited dozens of public executions in Aleppo and Raqqa during the bloody Syrian civil war that activists say has killed more than 190,000 people since 2011.
The report cited how the group's fighters have beheaded or shot civilians, mostly adult men, accused of violating their harsh interpretation of religious law, as crowds of people, including children, have looked on. The purpose, according to the commission, is "to instill terror among the population, ensuring submission to its authority."
On Monday, U.N. High Commissioner for Human Rights Navi Pillay said Islamic State fighters had likely killed up to 670 prisoners in Mosul. Pillay described other crimes that she said amounted to "grave, horrific human rights violations."
In its push to seize parts of northern Iraq, Islamic State fighters most recently caused the expulsion of tens of thousands of civilians who scrambled for safety on a desolate mountain top, where some died of exposure and thirst. The civilians, who belong to Iraq's ancient Yazidi minority, also reported that dozens of women were seized by the fighters and were being kept in schools in Islamic State strongholds. Their fate is unknown.
The U.N. commission report, which is investigating potential war crimes in Syria, also said Wednesday that the Syrian government of President Bashar Assad likely used chlorine gas to attack civilians.
It was the first time the U.N. assigned blame for the use of the chemical agent.
The report cited victims and medical workers who described symptoms caused by exposure to chemicals, and witnesses who reported chlorine-like smells immediately after government helicopters dropped explosive-filled canisters from helicopters in the provinces of Idlib and Hama eight times from April 11-29.
Chlorine is not banned under the chemical weapons convention, but the use of any toxic material as a weapon is illegal under international law.
"In Syria, it is total impunity," said commission member Carla del Ponte, a Swiss former war crimes prosecutor. "Crimes are committed each day, from all parties, and nobody's dealing with the criminal responsibility for those crimes."