KABUL: A little-known Islamist militant group claimed responsibility on Wednesday for gunning down a Swedish journalist in Kabul, but officials said the murder remained unexplained and that they were hunting for two suspects.

Nils Horner was shot at close range in an execution-style attack in an up-market district of the Afghan capital, a city that is regularly hit by Taliban suicide attacks.

Tuesday's killing was claimed by Fidai Mahaz, a small splinter group linked to the Taliban-led insurgency that fights against the US-backed government in Kabul.

“Nils Horner was killed in this attack. He was not a journalist he was a (British) spy,” said a statement on the group's website.

A spokesman for the mainstream Taliban militants denied any involvement in the killing, as well as any affiliation with Fidai Mahaz.

Taliban sources in neighbouring Pakistan described the group as specialising in criminal activity, targeted killings and kidnapping.

Afghan authorities said they were aware of the Fidai Mahaz claim, but declined to comment further.

“We are making a lot of effort to catch the perpetrators,” said interior ministry spokesman Sediq Sediqqi.

“Two people were possibly involved in the murder of Mr Horner, they are the main suspects and were seen by witnesses. We are trying to get their images from CCTV.”

The killing came less than a month before presidential elections in Afghanistan on April 5 and the withdrawal of Nato combat troops by the end of 2014 after 13 years of fighting the Taliban.

Foreigners have been targeted before at guesthouses, luxury hotels and embassies in the heavily guarded city, but few have been gunned down in the street in daylight.

On the day of Horner's death, many of Kabul's security forces were on duty at the funeral of Vice-President Marshal Mohammad Qasim Fahim.

Horner, 51, was an experienced Hong Kong-based reporter who had previously been in Afghanistan to witness the fall of the Taliban in 2001 and in Iraq during the war in 2003.

He was shot near a Lebanese restaurant where Taliban insurgents killed 21 people, including 13 foreigners, in a suicide attack in January.

“We like to refrain from speculating on any possible motive, there is a lot of rumours around,” Christian Nilsson, spokesman at Sweden's embassy in Kabul, told AFP.

“It is an on-going police investigation. We will have to see what they find out. We are in touch with the police.”

Security restrictions have been tightened for many of the diplomats, aid workers and journalists based in Kabul, where international staff often live and work in fortified compounds.