A total of 85 soldiers are now helping the US army train moderate Syrian rebels. (FILE PHOTO)

British soldiers are now on the ground in Jordan and Turkey training a Syrian rebel force that will go on to fight both Syrian dictator Bashar al-Assad as well as terrorist group Islamic State (ISIS).

A Ministry of Defence spokesperson told IBTimes UK on Tuesday that 85 British soldiers has already arrived in the region in early May and were helping the US army train the rebels, who have been vetted by the US military after being selected by volunteers.

Defence Secretary Michael Fallon announced in March that Britain would take part in the US-led programme, which aims to train 5,000 rebels per year for three years in the use of small arms, infantry tactics and medical skills spread across bases in both Turkey and Jordan.

The MoD said that Britain had no role in the selection of the aspiring rebel fighters taking part in the programme, and that all vetting was being carried out by the US military.

The decision comes some four years after the start of Syria's brutal civil war, which has already seen more than 200,000 people killed and many more displaced by fighting. Dozens of Sunni militias, including IS, control swathes of the country, battling the forces of Assad and Shia Hezbollah fighters. There have been widespread accusations of war crimes on both sides.

The lack of a moderate Syrian opposition has intensified with the decline of the Free Syrian Army (FSA) and the rise of more radical groups such as Jabaat al-Nusra. With Assad on one side and radical Sunni groups on the other, a middle ground has been increasingly lacking.

How the militia that is being trained by US and British soldiers will fit into Syria's complex patchwork of Islamist groups and pro- and anti-Assad militias is unclear, but the relative size of the force – 15,000 over three years – means it makes up a fraction of the numbers of other militias, not to mention the Syrian army, which has around 150,000 soldiers.

"It is symbolic more than it is meaningful. It is clearly a very ambitious project and, if it is going to bear fruit, it is going to need a lot more external investment from the US," said Julien Barnes-Dacey, senior policy fellow at the European Council on Foreign Relations (ECFR).

One of the reasons for the small scale of the project at present, he added, could be US concerns about the rebels they are training and where they will end up once they return to Syria. There could be a fear that once they have been trained at US expense, the rebels will simply join other militia groups in Syria that the West would be uncomfortable in supporting.

"There is a desire for the UK and France to move more forcefully but it is the US reluctance over these concerns that is delaying the whole process," Barnes-Dacey said.

The US and British trained rebels will not be the only "third force" embroiled in Syria's complex civil war. The so-called Army of Conquest -- a coalition of rebels backed by Saudi Arabia, Turkey and Qatar -- has made steady gains against regime forces in Syria, recently capturing a military base near Idlib.