IS releases new images of Mohammed Emwazi

As the Islamic State jihadist group comes under growing military pressure, its prodigious propaganda output has slowed somewhat but turned increasingly gruesome in a bid to keep in the headlines.

Since it announced a self-styled caliphate across Iraq and Syria last year, IS has become notorious for broadcasting its macabre tactics.  

The group's online network has taken advantage of social media's instant publishing power to circulate footage of mass killings and beheadings.

IS shot to global infamy in particular last year following the release of a series of English-language videos depicting the grisly executions of several Western hostages.

In November, US President Barack Obama referred to the group not as "masterminds" but rather "a bunch of killers with good social media."

But to maintain the media frenzy, IS has resorted to increasingly morbid and elaborate methods of killing. 

"The reason IS ups the ante is because they think people will be bored and they need or want to stay relevant," said Mia Bloom, a professor at Georgia State University. 

Some of IS's more grisly ways of killing captives include firing rockets at them or tying them to ancient artefacts that the jihadists then blew up -- as happened in the historic Syrian town of Palmyra.

In a recent video from Iraq, it eschewed its usual before-and-after images of beheadings in favour of grisly footage showing the entire execution process.

The group has also claimed spectacular attacks abroad, including the coordinated assault in Paris that left 130 dead and the downing of a Russian airliner that killed all 224 people on board. 

"The graphic videos are attracting a personality type, people who want the real life 'Call of Duty,' people who have violent pasts and want to reinvent themselves," Bloom said, referring to a popular video game.