By Andrew Heavens

KHARTOUM, Jan 5 (Reuters) - U.S. security agents are investigating reports that a previously unknown militant group was behind the killing of a U.S. aid officer and his Sudanese driver in Khartoum, an embassy official said on Saturday.

John Granville, a 33-year-old officer with the U.S. Agency for International Development, was shot and killed while returning from New Year's celebrations in Sudan's capital early on Tuesday.

His driver Abdelrahman Abbas Rahama, 39, was also killed in the attack, rare in the capital considered one of the safest in Africa.

On Friday an Islamist Web site posted a message from a group calling itself Ansar al-Tawhid in Sudan (Companions of Monotheism), claiming responsibility for the murders.

Walter Braunohler, a spokesman for the U.S. embassy in Sudan, said FBI agents working with Sudanese authorities in Khartoum were aware of the claim.

"Everything is being looked into. What we are really interested in are facts," he told Reuters.

Braunohler said agents from the FBI and the U.S. State Department's Bureau of Diplomatic Security had been arriving in Khartoum over the past few days to investigate the attack.

"They are here to assist and support the embassy," he said. "The embassy is working with the Sudanese authorities. It is hoped that they will have some answer soon."

Diplomats in Khartoum speaking on condition of anonymity said it was still unclear what motivated the shooting. "It was a huge shock," said one. "No one expects this sort of thing to happen in Khartoum."

Khartoum's reputation was dented in August when Sudanese security services said they had uncovered a plot to bomb Western embassies in the capital.

Britain shut down the consular service in its Khartoum embassy after the shooting on Wednesday and Thursday and urged its citizens to be vigilant while travelling around the city.

"We are taking the incident very seriously," said a British embassy spokesman. "Diplomats are keeping a low profile at the moment. We really don't know what was behind the attack. It is too early." He said the embassy was hoping to reopen its consular service to the public on Sunday.

The current travel advice for Sudan published by Britain's Foreign and Commonwealth Office says there is a "high threat" from terrorism in Sudan. "Attacks could be indiscriminate, including in places frequented by expatriates and foreign travellers," its website said.

Sudanese officials initially said Granville had not been killed in a terrorist attack. A spokesman from the country's Ministry of Foreign Affairs said officials would be meeting on Saturday to discuss Ansar al-Tawhid's claim of responsibility.

It was not possible to verify the authenticity of the statement which was posted on a Web site commonly used by Islamist groups, including al Qaeda leaders.

The wording did not appear in the "statements and news" section of the site usually used for the publication of Islamist claims, statements and videos.

Instead, it appeared in a password-protected comments section, accessible to signed up members of the Web site. (Editing by Cynthia Johnston and Mary Gabriel)

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