The FBI has announced it believes it has identified 'Jihadi John', the executioner seen in the ISIS hostage videos

The FBI has identified the ISIS executioner who appears in the beheading videos of US journalists James Foley and Steven Sotloff, and British aid worker David Haines.

Agency director James Comey told reporters today that he knew who the militant was but would not be releasing his identity or nationality to the media.

The executioner, who has a British accent and has been dubbed Jihadi John, is one of four British jihadis known as the 'Beatles' holding hostages in Syria.

He appears in the group’s execution videos wielding a knife – and taunts President Obama before carrying out the killings

Today British officials would not say if the identity had been shared with the authorities on London, and would not be drawn on whether any arrests are imminent.

A Home Office spokeswoman said: 'We won't be commenting on matters of security at this stage.'

Comey said the FBI believes that ISIS is 'looking to try to do something in the U.S.,' but added he did not believe the group is capable of a sophisticated or complex attack.

During questioning from reporters at the FBI headquarters Comey also spoke about Khorasan group, an al Qaeda affiliate who were targeted along with ISIS by American airstrikes in Syria earlier this week.

He said he was 'not confident at all' the threat from Khorasan militant group was thwarted by the airstrikes.

'We don't have complete visibility about their activities.

'We believe they've been working toward an attack, but it's hard to say whether it would come tomorrow, in three weeks, or in three months,' he said.

FBI Director James Comey (pictured) told reporters today that he knew who the militant was but would not be releasing his identity or nationality to the media. The executioner, who has a British accent, is one of four British jihadis known as the 'Beatles' holding hostages in Syria

Comey added the FBI is still working hard to identify two people in ISIS videos who appear to have American or Canadian accents.

'We have a big focus going on that right now,' Comey said.

He said about a dozen Americans are believed to be fighting with extremist groups in Syria at the moment.

He says more than 100 Americans have either tried to go to Syria and been arrested, gone successfully, or gone and come back.

The figure of 100 has often been misinterpreted to represent the number of U.S. citizens fighting in Syria.

Comey says all the Americans who have returned after fighting with extremist groups are either under investigation, under surveillance or have been arrested.

The director says he's not confident the U.S. has identified every American who had joined up with militants in Syria.

The masked man was seen in the execution videos of American journalists James Foley, 40, (right) and Steven Sotloff, 31 (left). Comey did not address whether U.S. believes man actually carried out the killings

British aid worker, David Haines, 44, was seen in a gruesome video with Jihadi John before his death. Today British officials would not say if the identity had been shared with the authorities on London, and would not be drawn on whether any arrests are imminent

Yesterday British Foreign Secretary, Philip Hammond revealed intelligence agencies were closing the net on Jihadi John.

Hammond, who just ten days ago admitted special forces didn't know where the Islamic terror group was holding its British hostages, said the investigation into tracking down the killer with the British accent was 'narrowing down the field'.

Speaking to American television network CNN at the United Nations summit in New York, Mr Hammond said: 'We are getting warm, we are working on all the leads.

'There's a big investigation and we are getting warm. We're narrowing down the field, but I don't want to say any more at this stage.'

Earlier this week it was revealed that US spy planes are flying above Britain monitoring telephone and computer signals in a bid to track down Jihadi John and those who are communicating with him.

The aircraft, manned by British pilots and carrying FBI agents, are equipped with technology so advanced they can detect heat coming off a keyboard when a button is pressed.

A source revealed Jihadi John, comes from a South London suburb about 10 miles from Central London.

The signals and data collected by the spy planes is being sent back to the US to be analysed, the Sunday Express reported.

An FBI source told the paper: 'Electronic footprints might help us pinpoint the location of the British IS executioner because we believe there are associates of his in the UK who are directly communicating with him.'

A formation of US Navy F-18E Super Hornets refueling having carried out airstrikes on targets linked to ISIS. Comey said he was 'not confident at all' the threat from Khorasan militant group was thwarted by the airstrikes

Strong presence: Military leaders have said about two-thirds of the estimated 31,000 Islamic State militants were in Syria. The U.S. has also been increasing its surveillance flights over Syria, getting better intelligent on potential targets and militant movements

International revulsion after the scenes were posted on the internet sparked a massive manhunt by British and American security services and the investigation has now focused on a district about ten miles from Central London.

It's also believed the planes are being used to study the movements of Jihadi John's suspected Midlands associates.

An intelligence services source said the delicate detection equipment on board the planes had previously been used successfully to work out a suspected terrorist's computer password because of the heat signature left on the keys.

The intelligence source said the surveillance plane had recently been used over Birmingham in relation to Junaid Hussain - who is suspected of being one of the so-called 'Beatles' holding hostages.

Defiant: A fluttering ISIS flag is flown over a hill in Tel Abyad, on the Syrian-Turkish border. The area, which is in the heart of the Raqqa countryside, has been the subject of numerous coalition airstrikes

The planes are understood to be military aircraft which are sometimes used by the police to find escaped prisoners or stolen vehicles.

The US Senate has also offered a £6 million reward to anyone providing information that could lead to the capture of Jihadi John.

A team of more than a dozen US investigators from an FBI Joint Terrorism Task Force flew to the UK, where they are working with MI5 and anti-terrorist officers from the Metropolitan Police Force to identify the man.

It is believed family members of Jihadi John have been interviewed by Met officers and an FBI agent.

The source said Jihadi John was known to the security services even before he went to Syria, but had been regarded as a 'low-level player'.