Graphic videos showing British and American hostages being murdered by Islamic State (Isis) fighters are stirring support among foreign jihadis who are excited by a new confrontation with the west, monitoring of Islamists' social media activity suggests.

Barbarous online films, such as the two-and-a half-minute video showing the killing of British aid worker David Haines released on Saturday night, are "turning on" jihadists in countries such as Tunisia and Libya who had previously reacted coolly to the civil war between the Sunni fundamentalists of Isis and the Shia minorities in Syria and Iraq.

Evidence from the Twitter, Facebook, Ask.fm and Instagram accounts of 450 foreign fighters in Syria and Iraq and others who follow them suggests the filmed murders and speeches attacking Washington and London appear to have made Isis's cause more glamorous to extremists abroad, according to the International Centre for the Study of Radicalisation (ICSR) at King's College London.

The response may explain why Isis fighters have released the videos and have threatened to kill another Briton, Alan Henning. Some analysts have asked why Isis leaders would invite stronger military intervention from the west, but postings on social networks used by jihadis suggest the tactic could be aimed at widening support for Isis.

"We have been monitoring the social media reaction and you can already see that the wannabe foreign fighters are excited by these killings," said Peter Neumann, director of the ICSR. "People in Tunisia and Libya are particularly interested in the prospect of fighting American and now British enemies. This is turning on people who were radicalised before this conflict started but weren't particularly excited by the Sunni-Shia battle. There are three types of reaction. There is pure jubilation, comments that America and now Britain are getting what they deserve, and the thought that this is not pretty, but that this is the kind of thing that happens with revolutions."

The man who beheaded Haines, a father of two born in Yorkshire and raised in Scotland, directly addressed David Cameron in the film, ridiculing him as "an obedient lapdog" of Washington.

"Your evil alliance with America, which continues to strike the Muslims of Iraq and most recently bombed the Haditha dam, will only accelerate your destruction," the masked man said.

There are already estimated to be more than 2,000 Isis fighters from Tunisia in Syria, but Neumann said the direct challenges to western leaders in the beheading videos were "an even stronger incentive for people who were sitting on the fence and thinking about going to fight".

Other analysts claimed the latest video was a sign of Isis's weakness after US air strikes. It drew a measured response from Cameron who promised "a calm, deliberate" process of driving back, dismantling and destroying Isis. But a former senior government adviser on terrorism said it should be met with immediate military strikes by British forces.

"I despair at the stupidity of these three beheadings," said Afzal Ashraf, a former senior Foreign Office official in Iraq, now a consultant fellow at the Royal United Services Institute. "There is no rationale to this. They are annoyed and upset they are being killed and so they are wheeling these poor guys out who they have held in some cases for two years and who they were hoping to get a ransom for. This tells me they are really hurting and the only thing they can do is kill an unarmed hostage in a very, very cowardly act, which shows just how weak they are."

He argued that paratroopers and special forces should be sent in to kill Isis fighters manning the remote checkpoints that guard the group's recently claimed territory.

"We have to send out a message that must be understood by not very smart people," he said. "I would get a few of our guys who would be only too happy to deliver David Cameron's compliments to those outposts. They could be there by supper, out by breakfast and be home in time for dinner."

But Julian Lewis MP, a member of parliament's intelligence and security committee, said such a response would be to grant Isis's wish for the west to launch "crusader-style attacks in Muslim lands".

"These disgraceful performances are acts of deliberate provocation," Lewis said. "The reason they are doing this is they are seeking to consolidate their position in the vanguard of political Islam … The very fact they are going to such lengths … shows they know the position they are trying to carve out is not secure."

Lewis called for an ideological counterattack in the form of a mass public information campaign aimed at rubbishing claims that the brutality of the Isis militants has any root in Islam. "We need a greater national effort to identify the doctrine and undermine it," he said.

That was echoed by Ghaffar Hussain, managing director of the counter-extremism thinktank the Quilliam Foundation, who called for a "multi-agency and cross-departmental approach to countering all forms of extremism and preventing radicalisation, whether violent or not, stemming from the acceptance that jihadist organisations recruit from a much larger pool of non-violent extremists".