UK 'urgently investigating' video showing apparent British ISIS fighter.

The hunt for man in ISIS beheading video

STORY HIGHLIGHTS

• UK officials are investigating a video showing an unmasked apparent British ISIS fighter
• A Downing Street spokesperson told CNN the video was released Friday
• PM David Cameron discussed the video with officials Saturday, the spokesperson said
• The video preceded another showing the beheading of Briton Alan Henning by ISIS

(CNN) -- British police are investigating a video released Friday that shows an unmasked and apparent British ISIS fighter, a spokesperson for Prime Minister David Cameron says.

"The police are urgently investigating the contents of the video, including possible terrorism offenses relating to it," the spokesperson said.

The video was released the same day ISIS used another video to make public the beheading of Briton Alan Henning, 47 and threaten the life of U.S. aid worker Abdul-Rahman Kassig, known as Peter Kassig before he converted to Islam.

Henning, a 47-year-old taxi driver from Manchester, northern England, was part of a team of volunteers who traveled to Syria in December 2013 to deliver aid to people affected by the country's devastating civil war.

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He was abducted by masked gunmen the day after Christmas, others in the aid convoy said.

ISIS blamed its killing of Henning on the UK's decision to join the U.S.-led bombing campaign against the group in Iraq and Syria.

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Henning family: 'Numb with grief'

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White House condemns Henning's killing

Officials from Britain's intelligence agencies, Foreign Office, Home Office, police and military updated Cameron Saturday on work to take down the video of Henning and find those responsible, the Downing Street spokesperson said.

"The PM was clear that we must keep doing all we can to ensure that these terrorists are found and brought to justice for their heinous crimes and we will keep working with our U.S. partners and those in the region to do this.

ISIS "brutality will not persuade us to change our approach. Indeed, the senseless murder of an innocent man only reinforces our resolve to defeat this terrorist organization and to eradicate the threat they pose to Britons -- whether those in the region or here on the streets of the UK," the spokesperson said.

UK jets began flying reconnaissance flights over Iraq a week ago, and on Tuesday dropped their first missiles on an ISIS heavy-weapon position and an armed pickup truck in Iraq, according to the UK Defense Ministry.

READ: Who is ISIS?

Henning appears to be the fourth Westerner to be beheaded on camera by ISIS.

This summer, ISIS beheaded American freelance journalists James Foley and Steven Sotloff -- showing their gruesome killings in videos posted online. ISIS then claimed its first British victim, aid worker David Haines, according to video that appeared online on September 13.

Henning's family had pleaded for his release and said in a statement it was "numb with grief" at news of his death.

"It is the news we hoped we would never hear. As a family we are devastated by the news of his death. There are few words to describe how we feel at this moment," his wife, Barbara, said in a written statement released Saturday by the United Kingdom's Foreign Office.

"We always knew that Alan was in the most dangerous of situations but we hoped that he would return home to us. That is not to be."

UK authorities believe at least 500 British citizens have gone to Iraq and Syria, many of them to fight with ISIS and other Islamist groups -- and that most will try to return, bringing their extremist views with them.

Cameron last month laid out new measures to tackle the threat posed by would-be UK jihadists, days after Britain raised its terror threat level from "substantial" to "severe."

READ: ISIS video of 4th Westerner killing similar to others, but doesn't run as long