The Islamic State has released a video that appears to show the execution of David Haines, a 44-year-old British aid worker kidnapped by the militant group last year in Syria. The video was released just one day after Haines' family issued a plea for his captors to contact them in a statement released by Britain's Foreign and Commonwealth Office.

Haines was previously shown on camera during the execution of 31-year-old freelance journalist Steven Sotloff in a video that was posted online September 2 and verified as authentic by the US National Security Council on September 3. The latest video was first reported by SITE Intelligence Group, which tracks jihadist activity online.

The 2:27 minute video is titled "A Message to the Allies of America," and bears similarities to previous videotaped executions released by the Islamic State, including the logo of al-Furqan productions, a media group associated with the militants. The footage was initially posted on an IS-affiliated Twitter account.

The video opens with British Prime Minister David Cameron making a statement about Iraq. The video then cuts to Haines and his captor, a masked man shrouded in black holding a knife. Haines declares that Cameron "entered voluntarily into a coalition with the United States against the Islamic State," and is "entirely responsible" for his execution.

Cameron acknowledged the video in a statement posted on Twitter and vowed to "hunt down" those responsible.

We will do everything in our power to hunt down these murderers and ensure they face justice, however long it takes. — David Cameron (@David_Cameron)September 13, 2014

The murder of David Haines is an act of pure evil. My heart goes out to his family who have shown extraordinary courage and fortitude. — David Cameron (@David_Cameron)September 13, 2014

The masked militant also threatens to kill Alan Henning, another Briton currently being held hostage by the group. The video states that the execution of Haines is retribution for Cameron working with the US to arm Iraqi peshmerga forces fighting against the Islamic State, and for recent airstrikes targeting Islamic State positions near Iraq's Haditha Dam.

"Your evil alliance with America which continues to strike the Muslims of Iraq and most recently bombed the Haditha Dam will only accelerate your destruction," the masked militant says. "And playing the role of the obedient lapdog, Cameron, will only drag you and your people into another bloody and unwinnable war."

President Obama issued a statement condemning "the barbaric murder" of Haines.

"Our hearts go out to the family of Mr. Haines and to the people of the United Kingdom," the White House statement said. "The United States stands shoulder to shoulder tonight 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, and to degrade and destroy this threat to the people of our countries, the region and the world."