The Foreign Office is investigating reports that two British nationals have died while fighting in Syria.
The Britons are believed to have been fighting for The Islamic State in Iraq and the Levant (ISIS), a jihadist group active in Iraq and Syria.
Shiraz Maher, senior fellow at the International Centre for the Study of Radicalisation (ICSR) at King's College London, said on Twitter: "I t appears two British citizens have died in Syria while fighting for ISIS. They were fighting Jabhat al-Nusra at the time."
A Foreign Office spokesman said: " We are aware of reports and are looking into them.
"For some years FCO travel advice has advised against all travel to Syria, and all UK consular services in Syria were suspended some time ago. We continue to advise all British nationals in Syria to leave by whatever means.
"Even people travelling for well-intentioned humanitarian reasons are exposing themselves to serious risk, including being targeted for recruitment by terrorist groups. The best way to help the Syrian people is to donate to UK-registered charities that have ongoing relief operations."
Around 400 Britons are thought to have gone to Syria over the last two years, authorities believe, with an estimated 20 having died.
The number of Britons arrested after returning from Syria is also thought to have increased, with people detained for travelling to the war-torn country to fight or for helping others do so.
Abdullah Deghayes, 18, from Brighton, is believed to have died in Kassab, in Latakia province, last month after leaving the UK in January.
His father, Abubaker Deghayes, learned of his son's death via Facebook, and pleaded for his other two others sons, Jaffar, 16, and 20-year-old Amer to return from Syria.
He insisted his three sons were not "terrorists" but had travelled to Syria to defend "those who are weak".
Abdullah Deghayes, who was due to go to university in Brighton, is the nephew of Omar Deghayes, who was held by the United States as an enemy combatant at Guantanamo Bay detention camp between 2002 and 2007 after he was arrested in Pakistan.
Photographs were published in January of two British brothers, named in reports as Akra and Mohamed Sebah from north London, who were believed to have died in battle in the war-torn country in September.
Other Britons have died after going to help with humanitarian relief.