Islamic State militants beheaded British aid worker Alan Henning in a video posted on Friday, triggering swift condemnation by the British and US governments.

The footage on YouTube, highlighted on pro-Islamic State Twitter feeds, showed a middle-aged man in an orange jumpsuit kneeling next to a black-clad militant in arid scrubland, similar to past Islamic State beheading videos of two American journalists and a British aid worker.

As in previous videos, Henning appears to read from a script before he is killed. "Because of our parliament's decision to attack the Islamic State, I, as a member of the British public, will now pay the price for that decision," he says.

A male voice with a British accent says, "The blood of David Haines was on your hands Cameron," in references to the slain aid worker and to Britain's prime minister. "Alan Henning will also be slaughtered, but his blood is on the hands of the British parliament."

Henning, a 47-year-old taxi driver from Salford in northern England, was part of an aid convoy taking medical supplies to a hospital in northwest Syria in December last year when it was stopped by gunmen and he was abducted.

In a posting on Twitter, Cameron said: "The brutal murder of Alan Henning by ISIL shows just how barbaric these terrorists are. My thoughts are with his wife and their children."

US officials said they had no reason to doubt the authenticity of the video, titled "Another Message to America and its Allies."

"The United States strongly condemns the brutal murder of United Kingdom citizen Alan Henning," President Barack Obama said in a statement.

"Standing together with our U.K. friends and allies, we will work to bring the perpetrators of Alan's murder - as well as the murders of Jim Foley, Steven Sotloff and David Haines - to justice," Obama said, referring to other captives killed by Islamic State militants.

Near the end of the one-minute, 11-second video, the man in black introduces another hostage identified as American Peter Edward Kassig. His parents later issued a statement confirming their 26-year-old son had been taken captive while doing humanitarian work in Syria.

"We ask everyone around the world to pray for the Henning family, for our son, and for the release of all innocent people being held hostage in the Middle East and around the globe," Ed and Paula Kassig of Indianapolis, Indiana, said in the statement.

Kassig had served in the US Army during the Iraq war before being medically discharged, the family said. Pentagon records show he spent a year in the army as a Ranger and was deployed to Iraq from April to July 2007.

After leaving the army, Kassig became an emergency medical technician and traveled to Lebanon in May 2012, volunteering in hospitals and treating Palestinian refugees and those fleeing Syria's nearly four-year civil war, the family added.

He was detained on Oct. 1, 2013, while traveling to the eastern Syrian city of Deir al-Zor while working for Special Emergency Response and Assistance, a non-governmental organization he founded in late 2012 and based out of southern Turkey to treat refugees flowing across the border from Syria, his family said.

While in captivity, he converted to Islam and took the name Abdul Rahman, a family spokeswoman said.

"DISGUSTING MURDER"

US National Security Council spokeswoman Caitlin Hayden confirmed Kassig was being held by Islamic State. "We will continue to use every tool at our disposal - military, diplomatic, law enforcement and intelligence - to try to bring Peter home to his family," she said.

The beheading of Henning marks the fourth such killing of a Westerner by Islamic State, which has faced air strikes by US, British, French and Arab fighter jets since seizing swaths of Iraq and Syria in waves of sectarian violence.

Earlier on Friday, before the Henning video was posted, a British Islamic State fighter identified as Abu Saeed al-Britani appeared in a separate video, urging British Muslims to travel to Syria and Iraq to join Islamic State.

In the video on YouTube, al-Britani, wearing a camouflage shirt and what appears to be a cast on his right arm, calls British and US military forces "cowards" in carrying out airstrikes instead of putting troops on the ground.

"So send all your forces. Send them all. Send all your reserves. Send all of your back-ups, for we'll send them back one by one in coffins," he said.

Britain, a close US ally, recently announced it was joining a US-led air assault against the Sunni militant group's targets in Iraq, after weeks of weighing its options.

Muslim groups across Britain, including some organizations that are highly critical of British foreign policy and blame Western interference for fanning the recent crisis in Iraq and Syria, had called in vain for Henning's release.

Henning's wife Barbara had called him a "a peaceful, selfless man" and appealed to Islamic State to release him.

Islamic State is believed to be holding fewer than 10 Western hostages in Syria. The remaining hostages include British journalist John Cantlie, who has appeared in three Islamic State videos. - Reuters