LONDON (AP) - Prime Minister David Cameron is summoning military and security chiefs for an emergency meeting Sunday in response to the beheading of a British hostage and a threat against another.

Britain's Prime Minister, David Cameron. (AP Photo/PA, Andrew Milligan)

The meeting comes after Islamic extremists released a video showing the beheading of British aid worker David Haines and threatening another with death.

Mike Haines, the victim's brother, said David Haines had been murdered "in cold blood." Britain's Foreign and Commonwealth Office said it saw no reason to doubt the authenticity of the video.

Haines is the third Westerner beheaded in recent weeks by the Islamic State group, which has seized vast swaths of territory in Syria and Iraq. The first two were U.S. journalists.

Islamic State extremists had threatened Haines' life in an earlier video released nearly two weeks ago.

The masked man shown on the latest video, which ends with footage of Haines' body, seems similar to the man in the earlier beheading videos. The tall man speaks with a similar British accent and — as in the two earlier videos — threatens the captive with a large knife brandished in his left hand.

The video shows Cameron condemning the Islamic State group. The man with the knife then condemns Britain's support of American action against the group and says the captive must pay with his life.

The 44-year-old aid worker's family had issued a plea to his captors the day before the latest beheading video was released. They urged the hostage-takers to contact them. The family said IS had ignored earlier attempts to open communications.

British officials had said they were doing everything possible to protect Haines. An earlier rescue bid led by U.S. forces had failed, however, and it is not clear Western agencies know the precise location of the hostage-takers.

Haines was kidnapped in Syria in March last year when he was working for the French aid group Agency for Technical Cooperation and Development, or ACTED, to help victims of the fighting there.

After his killing, Haines' family released a statement praising his passion for charitable work.

Mike Haines said his brother was "most alive and enthusiastic" when involved with humanitarian missions.

"His joy and anticipation for the work he went to do in Syria is for myself and family the most important element of this whole sad affair," Mike Haines said of his late brother. "He was and is loved by all his family and will be missed terribly."

Haines worked for humanitarian missions in extremely dangerous conflict zones, including Libya during its civil war and South Sudan shortly after it gained independence.

In addition to ACTED, he had worked for groups such as Handicap International, which helps the disabled during conflicts, and Nonviolent Peaceforce, which sends unarmed peacekeepers into conflict zones.

Haines had a teenage daughter in Scotland from a previous marriage and a four-year-old daughter in Croatia with his current wife, Dragana, who did not comment Sunday morning on the news of the killing.

Mike Haines said his brother had joined the military as an aircraft engineer with the Royal Air Force after attending school and working for the Royal Mail. After some time in the military, Haines worked with the United Nations in the Balkans "helping people in real need," Mike Haines said.

"There are many accolades from people in that region that David helped," Mike Haines said. "He helped whoever needed help, regardless of race, creed or religion."

The leader of Bosnia's Islamic Community, Husein Kavazovic, called on fellow Muslims Sunday to "show resolve to stop the murderers." He said Haines' family can be proud of his legacy.

British officials kept news of Haines' abduction out of the public eye for security reasons until IS showed him in a video nearly two weeks ago.

President Barack Obama said after the killing that the United States would stand with Britain in an expanded effort against the terror group.

"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," he said.

Germany and France, which is holding an international conference Monday to combat IS, also condemned Haines' killing.

"The odious assassination of David Haines shows once more the need for the international community to mobilize against the base and cowardly Daesh," French President Francois Hollande said, using the group's Arabic acronym.

German Foreign Minister Frank-Walter Steinmeier called the Haines killing "an abhorrent act of barbaric violence beyond all limits of human civilization" and said the Paris meeting comes at the right time.

There needs to be "internationally agreed action to effectively stop the flow of fighters and money," he said.

Some British lawmakers called for Britain to launch air strikes against Islamic State forces after the killing.

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