Two young British men have reportedly been killed by US air strikes in the Syrian border town of Kobane while fighting alongside Isis.
Known as Abu Abdullah al-Habashi, 21, and Abu Dharda, 20, the men from London were said by Isis sources and social media accounts associated with the group to have died sometime over the last few days.
A Foreign and Commonwealth Office spokesman said: "We are aware of reports of the death of two British nationals in Syria.
"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."
Al-Habashi, also known by his nom de guerre Abu Abdullah al-Britani, grew up in north London in a Christian Eritrean family and converted to Islam when he was 16.
Dharda comes from a British-Somali background and grew up in west London and he travelled to Syria in December 2013 via Turkey.
It was said that Dharda was questioned by counter-terrorism police at a British airport as he left but was allowed on his journey because they were satisfied with the explanation he gave for the purpose of his trip.
The two men's Twitter accounts (@therealHabashi and @abu_dharda34) were both last active on 7 November and it is believed that they were killed a short time after.
Their tweets confirm that they were in Kobane before their accounts ceased to be updated.
Many Somali brothers from the west are gaining Shahadah in kobane make dua im the first Eritrean..me and the ikhwa are going on a holiday :)
; Habashi Witness (@therealHabashi) November 7, 2014
On the same day, Abu Dharda tweeted that they were seeking to kill more Kurdish members of the PKK who are armed to fight Turkey for cultural and political independence.
There are hundreds of pkk dead from Kobane battles so far but inshallah soon those hundreds will turn to thousands.. expect more bloodshed
; Abu dharda #IS (@abu_dharda34) November 7, 2014
In August, BBC Newsnight reporter Secunder Kermani communicated with al-Habashi in an instant message conversation in which he said he had gone to Syria nine months earlier.
He is believed to have appeared in at least two Isis videos posted online and he claimed that he was "one of the few" to have fought in Iraq.
Abu Abdullah al-Britani also told the BBC that his family had tried to convince him to return home from the Middle East, but said he was happy there and that there was "no going back".
His mother was said to be devastated by his decision to join the militant group.
The Government believes more than 500 Britons have joined the conflict in Syria.
About 27 British people in total are understood to have died after joining Isis, also known as IS and ISIL.
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