ISIS executes British aid worker David Haines; Cameron vows justice

(CNN) -- British aid worker David Haines has been executed by ISIS militants, according to a video posted Saturday to a website associated with the group, making him the third Western captive to be killed by the Islamist extremist group in recent weeks.

The ISIS video post showing Haines' beheading called his execution "a message to the allies of America."

It is produced very similarly to the videos that showed the executions of American journalists James Foley and Steven Sotloff, the last of which included Haines and the threat that he'd be killed next.

The new video pictures a masked ISIS militant placing his hand on another captive, whom he identified as Alan Henning, a British citizen.

In a tweet, British Prime Minister David Cameron called "the murder of David Haines" an "act of pure evil."

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

Haines offers brief scripted comments on the video, as does the man who kills him.

Directing his remarks at Britain, the executioner -- who sounds like the man who killed Foley and Sotloff -- says, "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 and claim the role of the obedient lap dog.

"Cameron will only drag you and your people into another bloody and unwinnable war."

Retired Lt. Col. Rick Francona -- an Air Force veteran intelligence officer and CNN military analyst -- surmised that if ISIS planned to dissuade Britain for teaming up with the United States, the group will be disappointed.

"ISIS has just guaranteed British cooperation with the American on all phases of what we're going to be doing," Francona said. "... I think this is now a Western fight; it's not just a U.S. fight."

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