A previously unknown militant group has claimed responsibility for killing a U.S. diplomat in the Sudanese capital of Khartoum on New Year's Day.

Cars drive past a patch of blood-stained sand at the spot where a US diplomat and his embassy driver were killed in a pre-dawn shooting attack in Khartoum, Sudan, 01 Jan 2008

The group, calling itself Ansar al-Tawhid (Companions of Monotheism), said its members shot 33-year-old John Granville and his Sudanese driver Abdel Rahman Abbas.

It was not immediately possible to authenticate the claim, which was posted on a Web site used by Islamists.

Granville was being driven home on New Year's Day when another vehicle cut his car off, and assailants opened fire.

FBI agents and other U.S. investigators have traveled to Sudan to investigate the shooting.

Sudan's foreign ministry has said the shooting was an isolated incident with no political connotations. Sudanese media reports quoted a government official earlier this week as saying there is no evidence the attack was an act of terrorism.

U.S.-Sudanese relations have been tense because of the violence in Sudan's Darfur region. The shooting happened one day after a joint United Nations-African Union force took over peacekeeping duties in Darfur.