The latest beheading by Islamic State is an act of barbarity and evil that shows why Australia is engaged in the international fight against the extremists, federal Attorney-General George Brandis says.

The Islamist group has released a video purportedly showing a masked militant killing British aid worker David Haines, who was taken hostage in Syria in March 2013.

His death is the third such execution in recent weeks, following the beheadings of US journalists James Foley and Steven Sotloff.

• Abbott sends 600 troops to fight ISIL

Senator Brandis says he is appalled by Haines’ murder.

“It just serves to demonstrate, not that it’s really necessary for there to be more evidence, how barbaric and evil these people are,” he told ABC television on Sunday.

“This is a problem for the world and that is why we in Australia are engaged. We need to be engaged.

“This terrorist entity, ISIL, is a fundamental threat to the Western world in particular.”

Leaders vow to catch killers

Condemning an act of “pure evil”, Prime Minister David Cameron vowed on Sunday that Britain would do all it can to catch the killers of Mr Haines.

As President Barack Obama offered US support for its “ally in grief”, Cameron faced growing calls to allow Britain’s military to help in Washington’s planned assault against the jihadist group that has seized parts of Syria and Iraq.

US Secretary of State John Kerry was in Paris to push for a broad coalition against IS and Australian Prime Minister Tony Abbott committed 600 troops to the US-led effort, denouncing IS as a “death cult”.

Obama slammed the latest attack as “barbaric” and said the US “stands shoulder to shoulder tonight with our close friend and ally in grief and resolve”.

Britain is yet to join US air strikes against IS in Iraq but has offered to arm Kurdish peshmerga fighters battling the militants in the north of the country, a move cited in the latest video as a reason for revenge.

Family pays tribute to a ‘good brother’

Mr Haines’s brother Mike paid tribute to a “good brother… who was recently murdered in cold blood.”

“He was, in the right mood, the life and soul of the party and on other times the most stubborn irritating pain in the ass,” he said in a statement.

“He was and is loved by all his family and will be missed terribly.”

Mr Haines, 44, was taken hostage in Syria in March 2013 and was threatened in a video released this month showing the beheading by an IS militant of US journalist Steven Sotloff.