Former infantryman James Hughes from Reading, and Jamie Read from Lanarkshire, are said to be in Rojava, northern Syria

A former British infantryman who served in Afghanistan is among a growing cohort of Britons joining the ranks of westerners travelling to Syria and Iraq to fight Islamic State (Isis) militants, the Observer has learned.

James Hughes, from Reading, Berkshire, is understood to be in Rojava, northern Syria, helping to defend the beleaguered city of Kobani as a de facto “mercenary” fighting on behalf of the Kurdish People’s Protection Units, the YPG.

According to his Facebook profile, Hughes served in Afghanistan three times and left the army this year after five years’ service. He appears to be fighting Isis forces alongside his friend Jamie Read, from Newmains, north Lanarkshire, whose Facebook page reveals that he trained with the French army. He describes having been involved in fierce gunfights against jihadists last week.

Meanwhile, the Metropolitan police are investigating the whereabouts of a 17-year-old woman from Haringey, north London, who travelled by Eurostar last week and was last seen in Belgium believed to be making her way to Syria, potentially the first known case of a British female fighter joining the struggle against Isis. Officers are looking into whether the teenager, of Kurdish descent, is planning to offer humanitarian assistance or join the ranks of the Kurdish YPJ, or Women’s Defence Units, which is battling Isis forces in Kobani.

The development highlights the dynamic of British nationals fighting one another in the strategic border city. Another two Britons – both from London – have reportedly been killed fighting for Isis in Kobani during the past two days. Abu Abdullah al-Habashi, 21, and Abu Dharda, 20, were thought to have died in the latest ongoing offensive by jihadists to seize the city from the YPG, which has lost more than 300 fighters there. Both Hughes and Read are serving with the YPG, which is backed by US and international coalition air strikes and Kurdish peshmerga forces.

The Britons appear to have been recruited by an American called Jordan Matson on behalf of the “Lions of Rojava”, which is run by the Kurdish YPG movement and whose Facebook page urges people to join and help “send [the] terrorists to hell and save humanity” from Isis.

Matson, who has been wounded in fighting against Isis, confirmed that Hughes and Read were with him, sending an invitation to the Observer: “U can travel to Rojava n meet them.”

On Facebook, Read outlines that he has been in fighting in northern Syria, writing on Thursday that the “shit hit the fan my ass was going 5 to 10 lol”, to which Matson replied: “It’s always interesting the first time you have a bullet fly past your head.”

It appears that Read arrived in the region recently, after undergoing training last month in the Czech Republic. Another Facebook message, on 5 November, states: “It looks like all the hard work has payed off I got my good news, most of you know what i’m doing for those that don’t you will have to wait haha can’t really say on here but all I can say is this time next week i will be living the dream.” A picture shows him alongside Matson in full combat gear.

The Kurdish rights campaigner Mark Campbell said that he had become aware of Read and Hughes enrolling with the YPG in Rojava and of other Kurds in Britain travelling to Syria and Iraq. Aman Banigrad, of London’s Kurdish Community Centre, said: “Some are travelling for humanitarian reasons, but others are going to the frontline with the YPG. People have been killed; one of our members lost a cousin fighting in Kobani two weeks ago.”

Kurdish sources estimate that dozens may have gone from Britain to the Middle East, with an unknown number killed in action. The Home Office said that it does “not hold data on British nationals fighting with the Kurds in Syria/Iraq”. Experts estimate that about 500 Britons have travelled to Syria and Iraq to fight for jihadists.

The developments follow reports from Kobani of westerners taking up arms against the militants, including claims that a number of European biker gangs had ridden to Syria and are helping to bolster the resistance. A Canadian woman – 31-year-old Gill Rosenberg – was recently identified as the first foreign female to join the Kurds battling the Islamic State in Syria.

David Cameron has insisted that there is a fundamental difference between fighting for the Kurds and joining Isis.

Although the Home Office states that taking part in a conflict overseas could be an offence under both criminal and anti-terrorism laws, it clarifies: “UK law makes provisions to deal with different conflicts in different ways – fighting in a foreign war is not automatically an offence but will depend on the nature of the conflict and the individual’s own activities.”

When Cameron was asked in September how volunteers with the Kurdish authorities and Isis fighters could be identified when returning to the UK, he said that “highly trained border staff, police and intelligence services” would be able to discern the difference between Islamic extremists and those fighting them.

The prime minister recently outlined new powers to prevent British jihadis from returning to the UK unless they agreed to strict controls. The UK is also directly arming Kurdish forces fighting Isis militants in Iraq.