A British man who claims he has been fighting alongside jihadi militants in Syria has told the BBC that the UK is “pure evil” and that he will not return until he can “raise the black flag of Islam” over Downing Street and Buckingham Palace.

BBC 5 live broadcast an interview with a man calling himself Abu Osama, who claimed that he had received military training and taken part in bomb-making and fighting with the extremist al-Nusra Front, an group which is linked to al-Qa'ida and banned in the UK, for the past year.

The man, who spoke in a West Yorkshire accent, said that he had been fighting for the establishment of a caliphate – which he referred to by the Arabic term Khilafah – across the Islamic world.

Download the new Independent Premium app Sharing the full story, not just the headlines

Speaking to interviewer Nicky Campbell, he said: “I have no intention of coming back to Britain, because I have come to revive the Islamic Khilafah. I don’t want to come back to what I have left behind. There is nothing in Britain – it is just pure evil.”

“If and when I come back to Britain it will be when this Khilafah – this Islamic state – comes to conquer Britain and I come to raise the black flag of Islam over Downing Street, over Buckingham Palace, over Tower Bridge and over Big Ben.”

The BBC said his claims have not been officially verified.

The man said he initially faced opposition from his family over his decision to go to Syria, but that “they can understand that this is a good cause.”

“They are a bit scared but I tell them we will meet in the afterlife,” he said.

“This is just a temporary separation. They said, ‘We understand now what you are doing’, and my mother said, ‘I have sold you to Allah. I don’t want to see you again in this world’.”

The interview was broadcast against growing concerns of home-grown involvement in terrorism, after Britons appeared in a propaganda video for insurgent group Islamic State in Iraq and Greater Syria (Isis).

Security services believe an estimated 500 Britons have left the country to fight in Syria, according to the BBC.

Religious leaders have called on British Muslims not to travel to Syria and Iraq, amid ongoing fears that people are leaving the country to take part in fighting.

Daily coronavirus briefing No hype, just the advice and analysis you need Enter your email address Continue Continue Please enter an email address Email address is invalid Fill out this field Email address is invalid Email already exists. Log in to update your newsletter preferences Register with your social account or click here to log in I would like to receive morning headlines Monday - Friday plus breaking news alerts by email Update newsletter preferences

An open letter signed by more than 100 imams released to coincide with the holy month of Ramadan urged British Muslims “not to fall prey to any form of sectarian divisions or social discord” and to offer help “from the UK in a safe and responsible way”.

Qari Mohammed Asim, imam at Leeds Makka Mosque, who helped organise the letter, said: “Here in the UK we are Sunnis and Shias, brothers in Islam – and brothers in Britain too. The conflict in Syria and Iraq can never change that, no matter how bitter the fighting.”

“We urge members of the community to echo this sentiment by adding their signature to this letter, joining the many others that have done so already.”

Additional reporting by Press Association