(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."
Haines' family just sent public message to his captors
News of the gruesome killing came the same day that Haines' family released a brief message to his captors through the British foreign office.
In it, the family says, "We have sent messages to you to which we have not received a reply. We are asking those holding David to make contact with us."
A logistics and security manager for the Paris-based Agency for Technical Cooperation and Development, a nongovernmental humanitarian agency, the 44-year-old Haines was abducted in March 2013 near a refugee camp in Atmeh, Syria.
At that time, Haines was working to arrange for the delivery of humanitarian aid to people staying at the camp. He had previously worked on aid operations for victims of conflict in the Balkans, African and other parts of the Middle East, according to an ACTED spokesman.
Haines' face became known to the world in the ISIS video, released September 2, in which he looks forward and kneels as a masked ISIS militant stands behind him.
The militant says in that video, "We take this opportunity to warn those governments who've entered this evil alliance of America against the Islamic State to back off and leave our people alone."
British officials said after the video's release that they had sent troops to try to rescue an unidentified British citizen "some time ago," but failed. They released no other details.
As to the latest video, the British Foreign Office spokesperson said the government is "offering the (Haines) family every support possible."
"They ask to be left alone at this time," the spokesperson said.
Obama: U.S. 'stands shoulder-to-shoulder' with Britain
Another hostage has been publicly killed by ISIS. Another one's life has been threatened. And fighters for ISIS -- which calls itself the Islamic State, in a nod to its efforts to establish a vast caliphate in the Middle East under its strict version of Sharia law -- are continuing to kill innocent civilians in Iraq and Syria.
The threat from ISIS has been brewing for some time. Having begun a decade ago as al Qaeda in Iraq -- only to be disowned earlier this year by al Qaeda, the group behind the September 11, 2001, terrorist attacks, for its heavy-handed tactics -- ISIS has taken advantage of instability in Syria and Iraq to become one of the most prominent and feared groups in the Middle East.
ISIS, which is also known as the Islamic State in Iraq and the Levant or ISIL, managed to become one of the most successful rebel groups working to overthrow Syrian President Bashar al-Assad. It also rampaged through much of Iraq, thwarting seemingly overmatched Iraqi troops and massacring those who did not subscribe to its extreme version of Islam.
It wasn't until last month -- albeit before Foley's killing -- that the American military jumped into the fray.
"It's a bit like trying to predict an earthquake: You can see pressure building up on the fault lines but not knowing when it's going to materialize (or how) quickly it can disintegrate," Rep. Adam Schiff, a California Democrat, said Saturday night. "Those things are very hard to predict."
Partnering with the Iraqi military and Kurdish fighters, U.S. warplanes have been striking ISIS targets in Iraq regularly since August 8.
Earlier this week, President Barack Obama announced that U.S. airstrikes would go after the extremist group in Syria, and perhaps beyond.
"We will hunt down terrorists who threaten our country, wherever they are," said Obama, who insisted American troops wouldn't fight "on foreign soil," though they will play support roles. "That means I will not hesitate to take action against ISIL in Syria, as well as Iraq. This is a core principle of my presidency: if you threaten America, you will find no safe haven."
The President vowed the United States won't do it alone. To this end, U.S. Secretary of State John Kerry was in Egypt on Saturday seeking that country's help in the fight against ISIS.
Britain is one country, at least, that has stepped up -- even before Haines' killing.
That includes providing "urgent military support" at the Iraqi government's request, including heavy machine guns, body armor and nearly half a million rounds of ammunition to Kurdish fighters known as the Peshmerga.
The scale of that support, though, could ramp up now with one Briton executed and Henning's life in limbo.
To this end, 10 Downing Street announced early Sunday that Cameron will convene an emergency meeting of his top security officials to discuss what Britain will do next.
Obama released a statement late Saturday after what he called Haines' "barbaric murder," offering his support for the aid worker's family and his native Britain.
"The United States stands shoulder-to-shoulder tonight with our close friend and ally in grief and resolve," the President said.
"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."
CNN's Nic Robertson, Deborah Bloom, Michael Pearson and Phillip Taylor contributed to this report.