British special forces have been given the green light to launch raids deep into ISIS-held territory in Syria and Iraq, senior intelligence sources have revealed.

Amid growing alarm at the threat posed directly to Britain, the SAS has been given 'carte blanche' to kill or capture the terror group's leaders – including the masterminds behind the Tunisian beach massacre.

An elite force of up to 100 troops is expected to be committed to the secret war, working alongside US special forces and Navy Seal teams.

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An elite force of up to 100 troops is expected to be committed to the secret war, working alongside US special forces and Navy Seal teams

David Cameron last week pledged a 'broad spectrum' response to the murder of 30 British tourists by ISIS gunman Seifeddine Rezgui in the Tunisian beach resort of Sousse.

According to the Sunday Times the SAS and SBS will be working with MI6 and GCHQ, the government's eavesdropping service, to target Isis and other terror groups which pose a threat to Britain.

The operations will be planned and co-ordinated by permanent joint headquarters in Northwood, northwest London. Sources said each mission will probably require the approval of the prime minister.

One senior intelligence source told the Sunday Times: 'The SAS have been pushing to be more proactive for quite a while.

'In the past couple of years they have planned a number of operations in Syria when the chemical weapon threat was high but the missions were vetoed at the highest level because of the risks involved.

'The Tunisian attack has led to a rethink and has speeded up the inevitable use of SF [special forces] against Islamic State.'

Amid growing alarm at the threat posed directly to Britain, the SAS has been given 'carte blanche' to kill or capture the ISIS fanatics led by the group's leader Abu Bakr al-Baghdadi (pictured)

David Cameron last week pledged a 'broad spectrum' response to the murder of 30 British tourists by ISIS gunman Seifeddine Rezgui (pictured) in the Tunisian beach resort of Sousse

In May US special forces carried out a raid in eastern Syria that killed Abu Sayyaf, a top Isis leader. His wife was captured and a female slave from the Yazidi community was rescued.

Last week the Defence Secretary Michael Fallon revealed ministers were considering ordering bombing raids against ISIS strongholds in Syria as early as September.

Mr Fallon said it was 'illogical' for the RAF to be bombing ISIS in Iraq, but not joining the American-led assault on their bases in neighbouring Syria.

The Prime Minister paid tribute to the victims of the Tunisian terror attack during a minute's silence last week

He said: 'IS has to be defeated in both countries,' he said. 'It's evil in Iraq is being directed by its headquarters in Syria.'

Mr Fallon said the prospect on fresh bombing raids would increase if it emerges that the massacre of 30 British tourists in Tunisia last week was ordered by the ISIS leadership in Syria.

'If it does link back directly to IS in Syria then we will have to reflect with the rest of the coalition how best we deal with that,' he said.

The RAF has carried out more than 300 raids against ISIS positions in Iraq since MPs backed military action last September following the beheading of western hostages.

At the time, ministers reluctantly agreed not to extend the bombing into Syria after Labour, and many Tory backbenchers, signalled they would vote it down. Mr Cameron pledged that MPs would get another vote before bombing raids are launched in Syria.

But Labour is now indicating it may reconsider its position. And ministers believe the growing public revulsion about IS may persuade MPs to support a fresh military campaign in Syria.

The Prime Minister's official spokesman said 'a lot has changed' since last September, particularly in the wake of the killings in Tunisia.

Mr Cameron spoke on Monday of the need to 'crush IS in Iraq and Syria'.

Last night, Isis released a shocking video claiming to show teenage assassins shooting dead 25 opposition soldiers in the ancient city of Palmyra in Syria.

Isis said the men were soldiers captured in the city of Homs. They were shot in the head inside Palmyra's amphitheatre.