The images, obtained by the BBC, show around a dozen commandos riding long-range patrol vehicles and armed with sniper rifles, anti-tank missiles and artillery.
They were photographed near al-Tanf, where the UK is supporting the New Syrian Army (NSA) as it fights to push Isis back from strategic territory near the borders with Iraq and Jordan.
The rebel group has been repeatedly targeted by fighters from the so-called Islamic State using car bombs and ambushes in battles to control a border crossing and al-Hamdan Airbase .
British special forces were spotted alongside the NSA in June, shortly after around nine of the moderate rebels were killed in an Isis assault.
Unlike regular army troops, they do not need Parliamentary approval to be deployed and the presence of covert operatives in Syria has never been confirmed by the Government.
The NSA is understood to have received training, weapons and equipment from the Pentagon as well as intensive air support from the US-led coalition.
Russian planes hit its base with cluster munitions in two air strikes in June, prompting a diplomatic row after American fighter jets attempted to prevent the strike and instructions over a special communications channel to avoid mid-air confrontations were ignored.
Colonel Christopher Garver, spokesman for the anti-Isis Operation Inherent Resolve, said the NSA was attempting to drive militants out of the border city of al-Bukamal and cut Isis supply lines and the flow of foreign fighters along the Euphrates River.
It was among the groups receiving training at coalition-run camps in Jordan and has reportedly been equipped with state-of-the-art American weaponry.
The NSA, which opposes President Bashar al-Assad, is drawn from local Syrians and believed to include elite former special forces commandos who defected from the regime’s army.
The group said coalition air strikes hit Isis positions in al-Bukamal on Tuesday morning, “dealing a devastating blow to Daesh” by destroying 83 oil tankers.
On Sunday, NSA fighters said they successfully intercepted Isis fighters and blew up one of the group’s car bombs during an attempted assault on al-Tanf.
The RAF is continuing to provide air support in the region, with two Tornados targeting a convoy of Isis oil tankers near al-Bukamal on Sunday.
Britain is also supporting the predominantly Kurdish Syrian Democratic Forces group as it pushes Isis back near Manbij in northern Syria.
The Ministry of Defence announced the deployment of 75 military personnel to train Syrian rebels as part of a US-led programme last year.
It said thousands of opposition fighters would be equipped to “help them defend Syrian communities against Isis’ brutal attacks before leading offensives themselves” in Turkey and other countries in the region.
The MoD told The Independent it does not comment on special forces operations and would not confirm the NSA was among the groups it supports.
British special forces are also said to be fighting against Isis in Iraq and Libya
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