The Observer newspaper reported that Jamie Read, from North Lanarkshire in Scotland, and James Hughes, from Reading in central England, had been recruited by former US soldier Jordan Matson to join the "The Lions Of Rojava", a Kurdish movement vowing to "send terrorists to hell and save humanity".
Read told the BBC World Service that he was in Syria to help the Kurdish people, and denied reports that the fighters were mercenaries.
Both Read and Hughes, who completed three tours of Afghanistan, are reported to be in Rojava, northern Syria, fighting IS militants trying to take over the embattled city of Kobani.
Around 500 Britons are believed to be fighting on the side of the jihadists.
US fighter Matson last month told the BBC that there were around 15 Western fighters, but that there were "hundreds" of other veterans considering joining.
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"A lot of ex-military are very upset with what is going on in Iraq," he told the BBC.
"We were there for so long and so many died to bring democracy to that region and for that nation to just fall apart and for all those lives to be lost for nothing enrages a lot of veterans and they want to come here to complete that mission."
The group's Facebook page shows photographs of what it says are new recruits from the Netherlands and Germany, while Canadian newspapers have reported that one of their former soldiers, Dillon Hillier, had also joined the fight against IS.
The British government is currently grappling with what to do with jihadist fighters returning from Syria and Iraq, and said it would deal with those returning from fighting alongside Kurdish forces, known as People's Protection Units, on a case-by-case basis.
"People who commit, plan and support acts of terror abroad and seek to return to the UK will be prosecuted by the UK authorities," the Home Office said in a statement.