British and US security services were trying on Wednesday to identify the Islamic State (Isis) militant with a British accent who appeared in a video of the apparent beheading of a US journalist, James Foley.

The UK foreign secretary, Philip Hammond, said intelligence agencies were trying to unmask the fluent English-speaking militant in the propaganda footage. The White House expressed horror at the release of the video.

The tape showed a masked Isis fighter beheading a kneeling man dressed in an orange jumpsuit who is purported to be Foley, a photojournalist who went missing in Syria in 2012. The masked executioner spoke in English, and said the slaying came in response to the air strikes ordered by President Barack Obama against Isis 12 days ago.

Isis warned of further revenge – including on another man purported to be a captured US journalist, Steven Sotloff, who also appeared in the video. The man purported to be Foley was made to read a statement blaming the US for his own murder.

Foley has been missing in Syria since November 2012, where he went to report on the struggle to overthrow dictator Bashar al-Assad. He was initially thought to have been captured by forces loyal to the Assad regime. For a time, his family believed he was being held at a military-controlled facility outside Damascus.

Diane Foley, his mother, released a statement on Tuesday night, saying her son gave his life to expose the suffering of the Syrian people. She asked his kidnappers to release their other captives.

"He was an extraordinary son, brother, journalist and person," she said. "We implore the kidnappers to spare the lives of the remaining hostages. Like Jim, they are innocents. They have no control over American government policy in Iraq, Syria or anywhere in the world.

"We have never been prouder of our son Jim. He gave his life trying to expose the world to the suffering of the Syrian people," she said.

In the UK, Scotland Yard's counter-terrorism command, SO15, launched an investigation into the video as leading linguistics experts said the man sounded like he was from London or the south-east of England.

Hammond said: "All the hallmarks point to [the video] being genuine. We're very concerned by the apparent fact that the murderer in question is British and we are urgently investigating – agencies on both sides of the Atlantic – are first of all looking to authenticate the video, to make sure that it is genuine, and sadly it appears to be, and then to see if we can identify the individual in question."

Scotland Yard warned the public that viewing, downloading or disseminating the video within the UK might constitute a criminal offence under terrorism legislation. A spokesman said: "The Metropolitan police service counter terrorism command (SO15) is investigating the contents of the video that was posted online in relation to the alleged murder of James Foley."

Meanwhile, leading terrorism experts warned that the video was a highly orchestrated recruitment tool to lure "psychopathic individuals" to fight alongside Isis militants in Syria and Iraq.

Prof Peter Neumann, director of the International Centre for the Study of Radicalisation at King's College London, said the militant was chosen to front the video to cause maximum impact in the west. "This is significant because it signifies a turn towards threatening the west. They are saying we're going to come after you if you bomb us," he said.

Neumann said British fighters had been carrying out "horrific acts" like beheadings, torture and executions for a year and a half, but this appeared to be the first with a western victim.

He added: "They clearly wanted someone who spoke fluent English because they wanted it to create maximum impact, especially in the US, and because there are not that many Americans it was probably the best second option.

"They want this to have maximum impact on the west and for parts of it to be streamed on American television networks they needed an English speaker, so it was more about the English language than the nationality.

"It's not significant that British fighters have been beheading and torturing because that's been happening for a year and a half. That sort of horrific stuff is something British jihadis have been doing for some time. You will find a number of instances of British jihadis executing, torturing and beheading other people – and we know it's not just Brits but other Europeans doing it – and occasionally this has come to the surface.

"Most people beheaded before were not westerners so that's why this is different. The significant thing is that this was an American and was connected to a direct message that 'we are targeting you'."

Leading figures in the field of counter-terrorism said it would be possible for intelligence services to identify the militant, despite it being filmed in an unknown location with the fighter dressed head to toe in black.

Claire