Units of British SAS troopers are in Syria, backing up forces in the region targetting Islamic State (Isis).

The SAS and the SBS, its British Navy sister unit, are stepping up operations in Syria. The special forces units will work with MI6 – which has a large network of agents within the Islamist group – and will use intelligence from UK eavesdropping service GCHQ as to fight IS.

The SAS are now on standby to assist RAF jets which may be sent into the country following the massacre of 38 tourists, including 30 British nationals, in Tunisia on 26 June.

Past missions British and American Special Forces moved in on the Syrian border town of Kobane to drive out jihadists slaughtering Kurdish civilians.

British and US special forces identified IS leader Abu Bakr al-Baghdadi and directed an air attack that left him critically injured.

Elite troops in Iraq have raided eastern Syria and killed a top Abu Sayyaf leader.

Other missions in Syria involved the substantial threat of chemical weapon being involved, but were vetoed due to the risks involved.

The British special forces are said to have been granted free rein to capture or kill terrorist leaders who pose a threat to the UK and to direct airstrikes in IS-held territory.

A senior intelligence source told the Sunday Times that the SBS has been pushing to take on a proactive, rather than reactive position – meaning the elite naval unit will have full autonomy to attack terrorist networks that pose a valid threat to the UK.

The plan for heightened operations will likely be similar to the operations in Iraq, Syria and North Africa. Forces of 60-100 soldiers – including elite British troops, and members of US Delta Force and Navy Seals – will plan and carry out drone attacks against Isis, as well as guide air strikes in war-torn Iraq and Syria.

The forces will also analyse battle damage and execute strike operations against high-value targets, including the regional leaders of Isis.

RAF Tornado bombers and Reaper drones have flown hundreds of missions over Iraq since September, killing hundreds of fighters.

British planes are already fighting IS in Iraq, but the UK's Defence Ministry experts believe that bases in must also be targeted.

Defence Secretary Michael Fallon now wants British planes to target the extremists "at source" in Syria, if the House of Commons agrees.

Sources say one reason British troops have not been deployed to Syria publicly is because the UK is funding "legitimate" Islamist rebels fighting President Bashir Assad's brutal regime.