The Times reports that they have been defending a rebel unit against ISIS attacks, marking the first evidence of the UK troop's direct involvement inside Syria rather than training fighters in neighbouring Jordan.
First Lieutenant Mahmoud al-Saleh said British forces frequently crossed the border to help the New Syrian Army (NSA), comprised of former Syrian special forces, as it defends the south-eastern village of al-Tanf.
Al-Tanf border crossing located in the southeastern countryside of Homs at the Syrian Jordan border.
“They helped us with logistics, like building defences to make the bunkers safe,” he said.
The NSA snatched al-Tanf – which occupies an important location close to the Iraqi and Jordanian borders - from Isil in May.
The rebels now face regular attacks as the "jihadist movement" tries to take it back, Guardian reports.
“They attack us at all times, 3am, 5am, 4pm, 11pm. If you look at the timing of the assaults it’s clear they don’t want us to get any rest. They’re using missiles, mortars and many suicide bombers,” said Lt al-Saleh.
According to The Times, the group's base in the town was hit by a suicide attack last month and British forces helped with rebuilding it.
Since starting with a 2011 crackdown on anti-government protests, Syria's conflict has evolved into a complex, multi-front civil war that has left more than 280,000 dead and forced millions from their homes.
ISIS emerged from the chaos in mid-2014, seizing control of large parts of the country and neighbouring Iraq, declaring a fundamentalist Islamic caliphate and committing widespread atrocities.
Meanwhile American-backed fighters advanced on Sunday to within three miles of Isil's stronghold of Manbij in northern Syria, threatening a crucial jihadist supply line.
The assault by the Syrian Democratic Forces adds to the pressure on Isil as it faces another offensive by Russian-backed Syrian Army troops in its bastion province of Raqqa.
Supported by air strikes by the US-led coalition, the SDF alliance of Kurdish and Arab militias has made steady gains since launching the operation against Manbij last week.
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