The U.N. Security Council has condemned the beheading of British aid worker David Haines by Islamic State militants, calling it a "heinous and cowardly murder."
The Islamic State group posted a video Saturday showing the killing in the same manner as those depicting the beheadings of American journalists James Foley and Steven Sotloff.
The Security Council said in a statement Sunday the act "again demonstrates the brutality" of the militants and serves as a reminder that humanitarian workers are facing increasing dangers in Syria.
The council called on the Islamic State group, the al-Qaida-linked Nusra Front and all others associated with al-Qaida to release all of their hostages.
Earlier Sunday, British Prime Minister David Cameron vowed to destroy the Islamic State, saying Haines' killing is the "embodiment of evil."
He said Islamic State militants are not Muslims, but "monsters," and promised to hunt down the killers no matter how long it takes.
In the video, Haines is shown kneeling in the desert while a black-hooded militant with a British accent says the killing is an act of revenge for Cameron joining forces with the United States. Another British hostage is brought out with the killer promising he will be next.
President Barack Obama called Haines' murder "barbaric," and said the U.S. stands shoulder-to-shoulder with Britain in "grief and resolve."
A well-known Islamic cleric, Sheikh Awat Naqshbandi, told VOA's Kurdish service that beheadings are in no way Islamic.
He said militants have no place in Islam and that the religion bans what they are doing.
Haines worked for aid agencies in some of the world's harshest combat zones, including the Balkans, Libya, South Sudan and Syria.
He was working for the French-based group Agency for Technical Cooperation and Development when he was kidnapped last year.
His brother issued a statement saying Haines had a passion for humanitarian work.