David Cameron is breaking off from his holiday in Cornwall to return to London after the "shocking and depraved" apparent beheading of an American journalist by a British Islamist extremist, Downing Street said.

The announcement came after Philip Hammond, the foreign secretary, acknowledged on Wednesday that a Briton appeared to be responsible for the killing, which was shown in a video released by Islamic State (Isis) militants.

A Number 10 spokeswoman said: "If true, the brutal murder of James Foley is shocking and depraved.

"The prime minister is returning to Downing Street this morning. He will meet with the foreign secretary and senior officials from the Home Office, Foreign Office and the agencies to discuss the situation in Iraq and Syria and the threat posed by Isil [Islamic State] terrorists."

Intelligence officials in the US and the UK have been studying the Isis video. According to one government source, "there is no serious suggestion that it's a fake".

On Monday, as his holiday in Cornwall began, Cameron insisted there was no need for him to be in London to manage the Isis crisis because he was always "within a few feet of a BlackBerry". But his decision to go ahead with his West Country break – his third holiday of the year, by some counts – has been criticised in the media.

US journalist James Foley went missing in 2012 in northern Syria while on assignment for Agence France-Presse and the Boston-based media company GlobalPost. Photograph: Steven Senne/AP

Downing Street would not say when Cameron was planning to return to Cornwall to rejoin his family.

Earlier on Wednesday, during an interview on BBC Radio 4's Today programme, Hammond said that although further analysis of the Isis video would need to be carried out, the man shown committing the killing "on the face of it appears to have been a British person" and that this would not be surprising given the "significant number" of Britons fighting with Isis in Syria and Iraq.

Hammond said the involvement of the man, who spoke with a British accent, illustrated the extent to which Isis posed a direct threat to UK security.

He also said that the government would consider sending soldiers to Iraq to help train and build up the capacity of the Iraqi army to fight the Isis insurgency.

"This is a poison, a cancer, what's going on in Iraq and Syria, and it risks spreading to other parts of the international community and affecting us all directly," Hammond said.

The man shown being murdered in the Isis video is understood to be James Foley, a photojournalist who went missing in Syria in 2012. Hammond said the video appeared to be genuine, and that the killer could well be a Briton.

"We've been saying for a very long time that there are significant numbers of British nationals in Syria, increasingly in Iraq, and one of the reasons why what is going on in Syria and Iraq is a direct threat to our own national security is the presence of significant numbers of our nationals who may at some stage seek to come back to the UK with the skills, the tradecraft that they've learned working with these terrorist organisations, potentially posing a threat to our domestic security here in the UK," Hammond said.

Hammond said Britain was committed to helping the Iraqi government fight Isis and that, although the Iraqi government "has made it clear that it does not need and actually wouldn't welcome western boots on the ground", it did want help with surveillance and technological equipment.

Asked if Britain would send soldiers to Iraq to train Iraqi forces, Hammond said this was "certainly something that we would consider".

"The training doesn't have to be done in country," he said. "We're doing it with Libyans, we're bringing hundreds of Libyan soldiers over here and training them in training camps in the UK, but equally we could decide to put limited numbers of trainers into Baghdad for example.

"We could train Iraqi trainers to go out and train Iraqi security forces. We absolutely are prepared to consider requests for technical assistance, training support, advice, and so on."

Hammond said the British military had already provided surveillance support in connection with the recent humanitarian mission in Iraq and that "in principle" it could offer surveillance support in connection with the Iraqi army's fight against Isis.

He also refused to deny a suggestion that British special forces were already in Iraq, saying it was policy not to comment on special forces.

Hammond said that British security was threatened by Isis whether it failed in Iraq or succeeded. "If the Islamic State, so called, becomes established in an area of Syria and Iraq, it will undoubtedly use it for a base, for launching attacks on the west. It will undoubtedly send its fighters out to attack western targets," he said.

"Equally if it gets pushed back, some of these people will return to their countries of origin, not just the UK, all European countries, Australia, the US, other Arab countries. We will see these people going back and potentially carrying on their fight in our own homelands."

In his Today interview, recorded just two hours before Downing Street announced that Cameron was returning to London, Hammond defended the prime minister's decision to take a holiday. "We are all entitled to a break," he said. "We all operate a bit more effectively from having had one."

He also pointed out that Cameron was only "a couple of hundred miles down the road" from London.

Douglas Alexander, the shadow foreign secretary, said the killing of Foley had "shocked and outraged" the world.

"Suggestions that the attack may have been carried out by a UK national are particularly concerning and the government must now work with international partners to establish the facts and uncover any possible information about the perpetrator," he said.