In a bid to break the deadlock in the war-torn nation, Royal Marines will join the SAS to provide the New Syrian Army (NSA) with advanced training in neighbouring Jordan.
After the training has finished, NSA fighters will return to battle alongside SAS forces in Syria.
“Tours have extended and more are going to the region,” the Sun quoted an unidentified senior source as saying.
“The government is throwing absolutely everything at this, money and manpower, everything is being hurled at the problem.”
The NSA is a rebel force that mainly consists of Syrian Arab Army defectors.
The group was established in 2015 with US backing to fight IS in Deir ez-Zor province, which is almost entirely under the control of the jihadist group.
According to the Guardian, the NSA has had a “halting and uninspiring track record.”
The newspaper says the NSA’s most significant operation occurred in June this year when the group launched an attack on Al-Bukamal, a town on the Iraqi border that has long been a crossing point for foreign jihadists during the US occupation, and which is now being held by IS.
The attack failed, apparently due to the lack of sufficient air power backed by its Western allies.
Britain’s MI6 and France’s foreign spy service, the DGSE, are also training agents recruited from refugee camps in Jordan, it has emerged.
Once they are back inside Syria, the agents provide intelligence information on the civil war.
Preventing IS fighters from permeating NSA ranks has been a key issue, an unidentified source told the newspaper.
“The aim is to raise the level of training and capability for these forces.
“As we understand the vetting process has now been agreed, it has been a challenge, as many of these guys are ‘clean skins’ they have little background data that can be checked other than their word.
“But the Americans have established a simple but robust process that we will need to monitor and observe throughout the trainees’ time in the camps. The biggest threat is of infiltration by ISIS.”
On Monday, photographs of British special forces operating on the ground in Syria emerged.
It is believed to be the first time British forces have been photographed operating inside the country, where they are engaged in relatively small numbers.The troops were pictured driving the Al-Thalab, a high-mobility long range patrol vehicle designed for surveillance, reconnaissance, internal security and border patrol forces, as they moved around the perimeter of a rebel base close to the Syria-Iraq border.
The BBC reported the soldiers were working at the base in a defensive role, and a spokesman for the NSA acknowledged that British special forces had provided training, weapons and other equipment.
The special forces have a free-ranging role, operating in the border areas between the IS stronghold of Raqqa in Syria and the towns linking it to its northern Iraqi bastion, Mosul.
The UK Parliament approved an air campaign against IS in Syria but not ground troops.
However, special forces can be deployed wherever there is judged to be a threat to the UK.