Hundreds of British Muslims have travelled to Iraq to fight for the extreme jihadist group ISIS, terror experts have said.

The Government believes more than 500 British citizens have left the UK and gone to join rebel fighters in Syria, with many of them feared to have crossed the border to join the uprising in Iraq.

The ruthless ISIS (Islamic State in Iraq and Syria) extremists control large areas of land in Syria and are now turning their attention to Iraq and particularly Baghdad, having already taken the cities of Mosul and Tikrit.

Meanwhile, British security experts are also looking at the possibility that home-grown extremists trained by the ultra-violent group may bring terror back to the UK.

Scroll down for video

Britons are leaving the UK to fight with the ultra-violent ISIS group in Syria and Iraq, it was claimed today Bloodshed in Iraq: This video grab shows violence on the road to Baghdad as ISIS marched south today Carnage: Isis fighters, who took Mosul, pictured, earlier this week, are known for their violence and ruthlessness

One terror expert told MailOnline today: 'There is a fairly high chance that someone will attempt it.'

More than 500 British Muslims are believed to have already travelled to Syria to fight against President Assad, and it is feared many more will join them. Many of them are now feared to have crossed the barely-existent border into Iraq with the ultra-violent ISIS militia.

Charlie Cooper, researcher at the anti-extremist think tank, Quilliam Foundation, said: ' People are going from Britain and the EU to fight for ISIS - the estimate is more than 500 and that number will probably rise.

'If you want to go and fight jihad, ISIS, which holds a particularly extreme view of Islam and wants to create an extreme Islamic utopia, is a very attractive group to fight for.'

He added: 'Britons are going from Syria into Iraq because of this utopian promise. It is the only group in the world that has come close to establishing its version of an Islamic state, which is a big thing for people - including the British - who feel drawn to this kind of Islamic extremism.

'People who are starting to go out to Syria from Britain will see the huge gains that ISIS has made in Iraq in the last few days and they will be keen to join them because they see the potential.'

ISIS is cash rich, having looted hundreds of millions of dollars from the Mosul Central Bank when it took Iraq's second city earlier this week, and is helped as a force by its terrifying reputation.

Attractive to extremists: ISIS fighters preach an extreme form of Islam and want to create an Islamic utopia Terror: Footage reportedly taken by ISIS shows Islamist fighters randomly shooting pedestrians and motorists

Mr Cooper said: 'They took Mosul with 800 soldiers - against a reported 30,000 Iraqi soldiers. Their reputation precedes them.'

He said he believed the next week was crucial in establishing what ISIS' aims were, with the possibility that the group may try to secure its position in Iraq and stage a tactical retreat from Syria, or vice versa.

Today ISIS continued its march south towards Baghdad, shooting civilians alongside Iraqi soldiers, according to the UN. A video purportedly taken by ISIS militants emerged this morning showing gunmen carrying out indiscriminate drive-by shootings.

A spokesman for the Foreign Office said it was 'very concerned' with the escalating violence and was 'not going to take its eye off the ball'.

Mr Cooper warned that once the ISIS position was secure, there was a danger they may look to the west, with the possibility of British-grown, ISIS-trained extremists bringing their terror tactics back home.

He said: 'It's estimated that one in nine jihadists return home from jihad wanting to try to commit terror offences at home, and I would argue that they would look to the west.

'In ISIS they fight in battalions organised by nationality, the French together, the Belgians together, and so on, which is a clever tactic as it means when they return home they have an organised network of people to coordinate.

'I think what's happening out there is really, really worrying.'

Territory so far : The map shows the areas of Iraq and Syria currently controlled by ruthless ISIS forces

David Cameron's official spokesman acknowledged fears British citizens could be among the ISIS fighters in Iraq, saying: 'Our security services and all the relevant agencies will be monitoring those types of risks very closely.

'Clearly there is a very porous border between parts of Syria and parts of Iraq.

'As the Prime Minister has said, the greatest extremist activity and jihadist threats to the international community are in Syria.

'We need to keep these things under very close watch.'

Immigration and Security Minister James Brokenshire said: 'The police and security services are working to detect and disrupt terrorist threats from Syria and will take the strongest possible action to protect our national security.



'The Serious Crime Bill would extend the reach of the Terrorism Act 2006 so that UK-linked individuals and those who seek to harm UK interests, who travel overseas to prepare or train more generally for terrorism, can be prosecuted as if their actions had taken place in the UK.



'Syria is a dangerous place and the UK advises against all travel to the country. Even people travelling for well-intentioned humanitarian reasons are exposing themselves to serious risk, including being targeted for recruitment by terrorist groups. The best way to help Syrian people is by donating to UK-registered charities.'



A Foreign Office spokesman said: ' Even people travelling for well-intentioned humanitarian reasons are exposing themselves to serious risk, including being targeted for recruitment by terrorist groups.

'The police and security services are actively working to detect and disrupt any terrorist threat from Syria and individuals who travel there.

'People who are thinking about travelling to Syria to engage in terrorist activity should be in no doubt that we will take the strongest possible action to protect our national security, including prosecuting those who break the law.'

Mr Cameron has spoken to the secretary general of Nato about the security situation in Iraq, Downing Street said today, stressing that Mr Cameron's conversation with Anders Fogh Rasmussen did not relate to any possible Nato deployment of military resources.

British ISIS fighter reported 'Killed in action in Syria' appears in court

ISIS flag: Khawaja is accused of attending a training camp linked to ISIS

A British Muslim reported to have been killed in Syria appeared in a London court today charged with training to be a terrorist with an extremist Islamic group linked to ISIS.

Imran Khawaja, 26, of Southall, west London, who is accused of spending up to six months training with ISIS, appeared in court alongside his cousin, Tahir Bhatti, 44.

Bespectacled Bhatti, of Watford, Hertfordshire, is accused of driving to Bulgaria to colleck Khawaja, helping him buy an AK47 assault rifle, and other offences relating to aiding or abetting the alleged terrorist.

Khawaja, who was thought to have been killed in action when the terrorist group Rayat al Tawheed announced his death last week, appeared before Westminster Magistrates' Court today.

He was accused of attending a Syrian terrorist training camp run by Rayat al Tawheed, and of buying return tickets to Kurdistan, from where he is believed to have entered the war zone, on a compromised credit card.

Khawaja, believed to be known by the ‘battle name’ Abu Daigham al-Britani, allegedly received firearms training at the base run by associates of ISIS.

He and his cousin, a father of seven who runs a taxi firm, were arrested earlier this month by specialist counter-terrorism officers, Westminster Magistrates Court heard.

Khawaja is also accused of buying return tickets to Kurdistan on a compromised credit card, and is believed to have used this route to enter the war-zone.