BREAKING NEWS: ISIS Executes British aid worker David Haines

CNN) -- British aid worker David Haines appears to have 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' apparent 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.

When contacted by CNN, the British Foreign Office said that it was analyzing the video.

Alan Henning with an ISIS member in a frame taken from a video released by ISIS.

News of the apparent 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.

British hostage of ISIS was helping displaced Syrians, aid group says

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.

ISIS, which also calls itself the Islamic State, has killed thousands in Syria and Iraq as it presses a military campaign to establish itself as an Islamic caliphate. Witnesses report mass killings, beheadings and crucifixions.

The United States has been using airstrikes to support Iraqi and Kurdish forces battling ISIS, and is working to build a coalition to broaden the effort against ISIS. U.S. Secretary of State John Kerry was in Egypt Saturday seeking that country's help in the fight.