Hardline group claims responsibility for death of radio journalist in Kabul and accuses him of spying for UK government

A Taliban splinter group linked to the kidnapping of a New York Times journalist six years ago has claimed responsibility for killing a British-Swedish reporter in Kabul.

The Fidai Mahaz group said it had killed the radio journalist Nils Horner, 51, and accused him of being a spy for the UK government, in a statement posted on its website.

"[Horner] was not a journalist, he was a spy of MI6, he was a special worker of MI6. He was targeted by Fidai movement," said the brief English-language statement attributed to the spokesman Qari Hamza.

The group has only weak ties to the mainstream Taliban, and is even more hardline. It last made headlines nearly two years ago in a different incarnation – the Mullah Dadullah front – claiming responsibility for the assassination of the former Taliban official Arsala Rahmani, who at the time was trying to help broker a peace deal.

But it offered no proof of any role in Horner's shooting or evidence of any espionage activities; the statement was dated Tuesday but did not reach Afghan media until long after the victim's name and other details had been made public.

"This conflict is steeped in duplicity," said the academic and Taliban expert Michael Semple, who said it was impossible to say with any clarity who was behind the attack. Many militant groups have a pattern of using summary executions to intimidate civilians in Afghanistan and Pakistan.

"[Fidai Mahaz] certainly have this track record of trying to build themselves up," Semple said. "The publicity that they seek is no doubt part of the strategy of getting money. The group tries to develop itself by getting a high profile for bad actions."

The Fidai Mahaz group does have some history of targeting foreign journalists. Its leader, Haji Najibullah – who shares his name with Afghanistan's last communist president – is believed to be behind the 2008 kidnapping of the New York Times journalist David Rohde, who was abducted as he travelled with Afghan colleagues to interview a Taliban commander.

Other insurgent groups strongly denied any link to Horner's assassination on Tuesday, with one even telling the Guardian that although it targeted foreigners it did not include journalists on hit lists.

Afghan analysts were also puzzled by the staging of such a brazen attack in the heart of Kabul, just days after Horner arrived.

He had hoped to speak to survivors of a Taliban attack in January inside a nearby restaurant popular with foreigners, but there was no indication he had been chasing any sensitive angles on the widely reported story.

"It's a very strange issue because the area was under the control of security forces. He was trying to make a report on the Lebanese restaurant but instead was killed in the area," said the political analyst and former Taliban official Waheed Mozhdah, hinting at a possible foreign role.

"This area is full of security. What were these assassins doing in the area? It shows there were some other hands behind the curtain. Also in Kabul there are some other groups which have no connection with the Taliban but start to do terrorist activities."