London, Asharq Al-Awsat–The Islamic State of Iraq and Syria (ISIS) was facing worldwide condemnation on Sunday after releasing a video showing the execution of another Western hostage, this time from the UK.

ISIS released a video showing the beheading of British aid worker David Haines on Saturday night. In the video, Haines is shown dressed in an orange jumpsuit in a desert landscape, in similar format to two previous videos released by the organization in recent weeks, which showed the murder of two American journalists.

Haines, a 44-year old father of two, was abducted in Syria’s Idlib province in early 2013 while working for the French aid agency ACTED.

His killing was condemned by British Prime Minister David Cameron as an “act of pure evil.”

“We will do everything in our power to hunt down these murderers and ensure they face justice, however long it takes,” Cameron added.

The prime minister was due to chair a meeting of Cobra, the British government’s emergency response committee, on Sunday morning in order to discuss the response to the killing.

The release of the video came a day after Haines’s family issued a public appeal to ISIS to contact them. However, it is unknown precisely when Haines’s killing took place.

In the video, titled, ‘A Message to the Allies of America,’ one of Haines’s captors says he is being killed in retribution for the UK’s decision to offer arms and aid to Kurdish fighters battling ISIS in Iraq. It also follows US airstrikes on ISIS fighters around Iraq’s Haditha Dam on September 7, indicating he may have been killed sometime last week and the video of his death released later.

The release of the video was met with condemnation from the leaders of the states that have pledged to take on ISIS in Iraq, including the US.

US President Barack Obama issued a statement condemning what he said was “the barbaric murder” of the aid worker, and added: “The United States stands shoulder to shoulder with our close friend and ally in grief and resolve. We will work with the United Kingdom and a broad coalition of nations from the region and around the world to bring the perpetrators of this outrageous act to justice.”

The office of French President François Hollande also condemned the killing, issuing a statement on Sunday which said: “The heinous murder of David Haines shows once again how the international community must mobilize against [ISIS].”

Australian Prime Minister Tony Abbot said Haines’s death was evidence that ISIS “does not just do evil, but exults in doing evil.”

The Australian government announced on Sunday it was sending eight combat jets, an air-to-air refueling plane and 200 special forces troops to the United Arab Emirates to join in US efforts to combat ISIS in Iraq. France, meanwhile, is due to host an international conference on the security situation in Iraq on Monday, and has also signaled its willingness to join the US in taking military action in Iraq against the group.

However, all four states—the US, UK, France, and Australia—have ruled out putting “boots on the ground” in Iraq, with the exception of a small number of special forces troops and military advisers to assist Kurdish and Iraqi government troops.

In the UK, Haines’s death at the hands of ISIS was also denounced by Muslim leaders.

In a press statement released by the Muslim Council of Britain, the organization’s secretary-general, Shuja Shafi, said: “David Haines went out to the region to help the people of the region. That extremists chose to murder him shows once again the depravity of their warped ideology.”

“[ISIS] claim to be acting in the name of Islam. But there is nothing in our faith that condones such behavior.”

In a statement released via the UK’s Foreign and Commonwealth Office, Haines’ brother Mike paid tribute to his sibling, and said he was “murdered in cold blood.”

“David was most alive and enthusiastic in his humanitarian roles. His joy and anticipation for the work he went to do in Syria is for myself and [our] family the most important element of this whole sad affair,” he added.

“He was and is loved by all his family and will be missed terribly.”

As in previous videos released by ISIS, Haines’s murder is followed by a threat against another hostage, who is also believed to be a British national.