Two British men are thought to have travelled to Syria to join up with Kurdish fighters against the Islamic State (IS), according to reports.
The Observer newspaper said James Hughes, reportedly an ex-soldier, from Worcestershire, was understood to be in Rojava, in the north of the war-torn country, and helping other American and British fighters defend the besieged city of Kobani.
The paper also said a second Briton, named as Jamie Read, from Newmains, North Lanarkshire, was fighting against IS in Syria and cites a conversation it had on Facebook with an American, Jordan Matson, who is said to have recruited the Britons to the so-called Lions of Rojava group, aligned to the Kurdish YPG, or People's Defence Units, movement.
The Home Office warned against travelling to the war-torn country and said anyone who did was putting themselves in "considerable danger".
During a phone call Mr Read told the BBC: "We are here to help the Kurdish people, the YPG."
He added, "we are into a bit of trouble" before cutting the call short.
Meanwhile, Mr Hughes' sister told Sky News: "Obviously, I am very worried about him going out in such a dangerous country. But we are all very proud of him.
"He's just doing it to help others. That's very much in his nature. I think he definitely thinks of the potential danger, but he is fearless of many things.
"Not much fazes him, and if it does he hides it very well."
A spokesman for the men, Graham Penrose, denied the men were "mercenaries".
"Over the course of the last 12 months, the lack of assistance that the Kurds in particular have received in trying to protect themselves motivated the guys to see if there was some assistance they could lend," he told Sky News.
"They are not being paid, they are receiving no payment, there is no promises of payments, no payment in kind, they are not participating for any commercial gain. They are simply out there to help people who are in need of help.
"I personally, and the people who know them, feel they deserve a lot better than having themselves described as mercenaries. They are not mercenaries."
The Home Office spokeswoman said: " The UK advises against all travel to Syria and parts of Iraq. Anyone who does travel to these areas, for whatever reason, is putting themselves in considerable danger.
"The best way to help the people of these countries is to donate to registered charities that have ongoing relief operations, not by taking part in a conflict overseas, which can be an offence under both criminal and terror laws.
"People who commit, plan and support acts of terror abroad and seek to return to the UK will be prosecuted by the UK authorities. Whether a prosecution for an offence is justified in an individual case is a matter for the Crown Prosecution Service to decide after a full police investigation.
"A Counter-Terrorism Bill will be introduced later this week, which will include new powers to control the return of and impose strict conditions on British nationals we suspect have been involved in terrorist activity abroad."
An FCO spokeswoman: "The UK has advised for some time against all travel to Syria, where all UK consular services are suspended. As we do not have any representation in Syria, it is extremely difficult to get any confirmation of deaths or injuries and our options for supporting British nationals there are extremely limited."
It comes after two Britons, Abu Abdullah al Habashi, 21, and Abu Dharda, 20, both from London, are thought to have died in the Syrian border town while fighting for IS.
It is thought that 500 Britons have joined the conflict in Syria and around 27 are understood to have lost their lives after joining the jihadis.
But Labour MP for Birmingham Perry Bar Khalid Mahmood suggested the actual number fighting with IS could be much higher.
He told the Sunday Telegraph: "The authorities say there are 500 British jihadists be the likely figure is at least three or four times that. I think 2,000 is a better estimate. My experience in Birmingham is it is a huge, huge problem."
A statement purporting to be from the men on a Facebook page linked to the Britons said they intended to help "innocent people".
The post on the Terrorism and Conflict Research Center page said: "The definition of a mercenary is 'a mercenary is a person who takes part in an armed conflict who is not a national or a party to the conflict and is motivated to take part in the hostilities by the desire for private gain'."
"That is clearly not the case for those of you who know or have met James Hughes and Jamie Read. They are volunteers and brave men ... whose conscience has motivated them to apply their skills to assist innocent people who have been left to their own devices in the face of terror from IS and to report their experiences so that Western European audiences can understand the imperative of assisting the Kurdish nation resist IS."