Fears are growing that British extremists fighting alongside al Qaida in Syria could pose a terrorist threat when they return home.
Four British men have been killed in recent months while fighting with the terror group against government forces.
Three of the men, all thought to be from London, were killed as they attacked fighters loyal to President Bashar Assad, while a fourth died two weeks later when he was shot as he ambushed an enemy position, The Times said.
Another Briton has spoken out about why he has joined an al Qaida group fighting in Syria.
Ifthekar Jaman, 23, from Southsea, Hampshire, told the BBC's Newsnight he was engaged in jihad, or holy war, to help set up a state based on Islamic religious law.
Security experts fear that Britons who have travelled to fight in Syria - estimated by MI5 to number between 200 and 300 - may seek to radicalise others when they return to the UK.
Two men who returned from the war-torn country were arrested in London last month after allegedly being linked to a terrorist plot, The Times said.
Raffaello Pantucci, of the Royal United Services Institute, told the newspaper that deaths of British fighters in Syria could lead to an increased threat of a terror attack in the UK.
He said: "The likelihood grows of someone deciding that they want to punish the West for standing by as the death and destruction in Syria continue, as does the risk that groups on the battlefield might decide to distinguish themselves by using these foreign recruits to launch attacks in the West."
Shiraz Maher, from the International Centre for the Study of Radicalisation at King's College London, said: "Many Britons have made it clear they intend to stay in Syria and seek martyrdom but they are young and angry and that motivation could mutate and one day they could return to attack the West."
The man killed during an ambush of Assad's forces in August was Mohammed el-Araj, 23, from Ladbroke Grove, west London, The Times said.
Calling himself Abu Khalid, he was jailed for 18 months in 2010 after being arrested during a violent protest outside London's Israeli embassy.
He fought alongside the three other men who were killed by shelling two weeks earlier, along with nine others of different nationalities.
One of those three, Abu Hujama al-Britani, was pictured in a photograph with the caption "ISIS shahid from West London", a group closely linked to al Qaida, while a comment says he is from Hounslow in west London.
The British men killed were among a group of foreign fighters in Syria who in September spoke of their willingness to die fighting to make the country an Islamic state. In interviews they said that fears of them launching an attack in Britain were overstated.
The Foreign Office said it was looking into reports of the men's deaths, and said that "moderate Syrians have been explicit that they want aid, not foreign fighters".
Meanwhile the family of Ifthekar Jaman told the BBC they had watched on as he became radicalised over time, and that they understood why he had joined the jihad.
His brother, Mustakim Jaman, said: " He was always trying harder and harder to practise (Islam). He was always trying to be as strict as he can, he wanted to be the best Muslim...
"If he dies in his cause, then he's not died in vain, has he? He's doing a good deed."
Speaking via the internet from Syria, Mr Jaman said: "I am ISIS (the Islamic State in Iraq and the Levant). This is the group I am with. We are trying to establish the law of God, the law of Allah.
"This is the duty on me... all these people are suffering. Muslims are being slaughtered."
Mr Jaman told the BBC he did not pose any threat to the UK and had no plans to return.