(CNN) -- A radical Islamist group has claimed responsibility for the murder of Swedish journalist Nils Horner. The Fidai Mahaz group said on its website Tuesday that it had "targeted" the correspondent on the Wasir Akbar Khan 15 Road in Kabul.
The brief statement, attributed to the group's spokesperson Qari Hamza, asserted that the slain man was a spy and "special worker" of British intelligence agency MI6.
Neither the statement, nor any other part of the group's website, offered any proof that the splinter group was behind the killing, and officials have cast doubts on the veracity of the claim.
"It is an ongoing investigation," Sediq Sediqqi, a spokesman for the Afghan interior ministry, told CNN. "The Afghan police force are working very hard to find out who really was behind this.
"We cannot verify any claim by anyone or any group at this stage."
Previous statements posted on the site claim attacks on American and other NATO troops, and on Afghan security forces. The group also said it was responsibility -- under a previous name -- for the assassination of Canadian permanent resident Arsala Jamal, who was the governor of the Afghanistan's Logar Province.
The group's leader, Mullah Najibullah, also known by his nom de guerre Umar Khatab, is at odds with the Taliban leadership and has openly called them "traitors." His faction is opposed to the Taliban's attempts at detente.
Qari Yusuf Ahmadi, a Taliban spokesman, told CNN that he had no idea who the group was, and said that the Taliban had no relationship with it.
A dual British and Swedish national, Horner worked as a reporter for Sweden's Sveriges Radio, mostly covering Asia and the Middle East. The 51-year-old recently moved to Hong Kong and had visited Afghanistan several times previously.
He was gunned down in broad daylight on a Kabul street Tuesday. A Lebanese restaurant near the site of the shooting was attacked in January.
The brazen killing comes at a time when Afghanistan is preparing for next month's presidential elections, and ahead of NATO's planned withdrawal of troops by the end of 2014.
Masoud Popalzai in Kabul contributed to this report