The Islamic State group on Sunday released a video claiming the beheading of US aid worker Peter Kassig in a warning to Washington as it prepares to send more troops to Iraq.

The video also showed the gruesome simultaneous beheadings of at least 18 men described as Syrian military personnel, the latest in a series of mass executions and other atrocities carried out by IS in Syria and Iraq.

Washington said it was working to confirm the authenticity of the video.

"If confirmed, we are appalled by the brutal murder of an innocent American aid worker and we express our deepest condolences to his family and friends," National Security Council spokeswoman Bernadette Mehan said.

White House press secretary Josh Earnest said President Barack Obama had been briefed about the reports of Kassig's murder.

In the undated video, a man who appears to be the same British-accented jihadist who beheaded previous Western hostages stands above a severed head.

"This is Peter Edward Kassig, a US citizen," the black-clad masked executioner says, urging Obama to send more troops back to the region to confront IS.

"Here we are burying the first American crusader in Dabiq, eagerly waiting for the remainder of your armies to arrive," the militant says, referring to a northern Syrian town.

Kassig's family said they were awaiting official confirmation on the death of the former US soldier, who risked his life to provide medical treatment and aid to those suffering from Syria's civil war.

- 'Treasured son' -

"We are aware of the news reports being circulated about our treasured son and are waiting for confirmation from the government as to the authenticity of these reports," Ed and Paula Kassig said in a statement.

They urged the media not to publish or broadcast images released by IS, saying this was "playing into the hostage-takers' hands".

"We prefer our son is written about and remembered for his important work and the love he shared with friends and family, not in the manner the hostage-takers would use to manipulate Americans and further their cause."

Kassig would be the fifth Western hostage killed by IS in recent months, after two US reporters and two British aid workers were beheaded.

Sunday's video was substantially different from the previous IS recordings. Kassig was not shown alive in the footage, and no direct threats were made against other Western hostages.

British Prime Minister David Cameron said he was "horrified" by the "cold-blooded murder" which French President Francois Hollande called a "crime against humanity".

The 26-year-old, who converted to Islam and took the first name Abdul-Rahman, was captured last year and was threatened in an October 3 video showing the beheading of British aid worker Alan Henning.

The claim of Kassig's killing came as Washington prepares to double its military personnel in Iraq to up to 3,100 as part of the international campaign it heads against IS.

The Sunni Muslim extremist group has seized large parts of Iraq and Syria, declared a "caliphate" and imposed its brutal interpretation of Islamic law.

The latest video also showed a highly choreographed sequence of the beheadings of at least 18 prisoners said to be Syrian officers and pilots loyal to President Bashar al-Assad.

Jihadists are shown marching the prisoners past a box of long military knives, each taking one as he passed, then forcing them to kneel in a line and decapitating them.

"Today, we're slaughtering the soldiers of Bashar," the same black-clad militant said in the video.

"Tomorrow, we'll be slaughtering your (US) soldiers. And with Allah's permission, we will break this final and last crusade, and the Islamic State will... begin to slaughter your people on your streets."

- Jihadists 'don't represent Islam' -

Kassig founded an aid group through which he trained some 150 civilians to provide medical aid to people in Syria. His group also gave food, cooking supplies, clothing and medicine to the needy.

Burhan Mousa Agha, a Syrian friend who worked with Kassig in Lebanon, described him as a funny, dedicated and brave man who only wanted to help.

"I want to apologise to his family. I'm sorry that their son died in my country, trying to help," Agha told AFP.

"They are animals, less than animals, they don't represent Islam. Peter wasn't fighting anyone, he was teaching people how to save lives.

"I want to send a message to everyone that Peter was a hero, a real hero," Agha added.

Sunday's video came as IS suffered battleground losses in Iraq backed by US-led air strikes, with government forces on Saturday breaking the jihadists' months-long siege of Iraq's largest oil refinery.

The Syrian Observatory for Human Rights monitor said IS also executed 13 men in Raqa in Syria Sunday, accusing them of belonging to pro-regime sleeper cells.

Murdered US aid worker Kassig had 'calling' to help Syrians
Washington (AFP) Nov 16, 2014 - Peter Kassig, an American aid worker jihadists claim to have killed, had a "calling" to help Syrians suffering due to the devastating civil war in their country.

The 26-year-old former soldier disappeared on October 1, 2013 and was being held by the Islamic State group, which released a video Sunday claiming Kassig's execution and showing a masked militant standing over a severed head.

Kassig had been in the region since March 2012, when he traveled to Lebanon on a break from university, his family wrote on a Facebook page dedicated to raising awareness about his plight.

After witnessing the plight of those displaced by the war in neighboring Syria, he emailed a letter to family and friends telling them he was not coming home yet.

"I have tried to live my life in a way that displays what it is that I believe, but the truth is, much of my life I have only been searching for my calling, I had not yet found it," he wrote.

"Here, in this land, I have found my calling."

The young American, who was also trained as an emergency medical technician, went to Turkey and founded an aid group, his father, Ed Kassig, said in a video last month.

"He helped train 150 civilians in skills needed to provide medical aid to the people of Syria. His organization gave food, cooking supplies, clothing and medicine to those in need," Ed Kassig said as he pleaded for his son's release.

- Desolation and hope -

In an email to a friend in 2012, also posted on the Facebook page, Kassig wrote about the desolation of war and his hope for the future.

"How will I tell you of the walls riddled with bullets and the flowers that grow over them?" he wrote.

"War never ends, it just moves around... Loss and destruction in this land brings about only survival; the determination to press on and rebuild," he added.

"I can't be sure but I think I'm starting believe that as beautiful as it is to finish building a house, its a better feeling laying the first brick again after its been torn down..."

In a quotation on the Facebook page, he admits he at times wished for something different. The quote is attached as a caption to a photo showing Kassig brandishing a blood-clotting agent he obtained to help treat wounded in Syria

"The truth is sometimes I really think I would like to do something else, but at the end of the day, this work is really the only thing that I have found that gives my life both meaning and direction."

Kassig, who converted to Islam during his captivity and took the Muslim name Abdul-Rahman, said in a letter smuggled to his family his faith was intact.

"In terms of my faith, I pray every day, and I am not angry about my situation in that sense. I am in a dogmatically complicated situation here, but I am at peace with my belief."

- 'Scared to die' -

In an October 3 video showing British aid worker Alan Henning's beheading, the threat was made that Kassig would be next.

IS has justified killing Western hostages as retaliation for US-led air strikes on the group over swathes of territory it has seized in Iraq and Syria.

In the letter received by his family in June, Kassig wrote that he was "scared to die".

"But the hardest part is not knowing, wondering, hoping and wondering if I should even hope at all. I am very sad all this has happened and for what all of you back home are going through.

"If I do die, I figure that at least you and I can seek refuge and comfort in knowing that I went out as a result of trying to alleviate suffering and helping those in need."