(Updates with identity of slain U.S. official, adds details)

KHARTOUM, Jan 1 (Reuters) - Gunmen killed a U.S. government aid agency official and his driver in Khartoum on Tuesday, U.S. and Sudanese officials said.

The unknown assailants opened fire as the official from the U.S. Agency for International Development (USAID) was heading home in an embassy vehicle shortly after midnight on New Year's day, diplomatic sources said.

The driver was killed instantly and the official was taken to hospital where he died from his wounds.

Walter Braunohler, public diplomacy officer at the U.S. embassy, told Reuters: "Unfortunately, he just died".

Braunohler said it was too early to speculate about motives and declined to identify the slain U.S. official beyond saying that he worked for USAID.

Sudan's state news agency SUNA identified the official as 33-year-old John Granville and said he had died from multiple gunshot wounds.

Sudan condemned the attack and said the perpetrators would be pursued.

Washington has long had tense relations with Khartoum due to the conflict in Darfur, which U.S. President George W. Bush has labelled genocide. The Sudanese government rejects that charge.

The shooting came a day after Bush signed a law to make it easier for states, local governments, mutual funds and pension funds to cut investment in companies doing business in Sudan, particularly its oil sector, because of the Darfur conflict.

The U.S. government warned its citizens in Sudan in August that it had information "an extremist group" might target U.S. government interests or facilities.

Al Arabiya television said the attack took place in a main street in the capital Khartoum.

Sudan Foreign Ministry spokesman Ali al-Sadig called it an isolated incident and said the investigation was being coordinated with the U.S. side. No arrests have been made.

He did not say whether the attack was criminal or political, but he said investigators were considering all options and that he believed the U.S. official was fired on from another vehicle.

"Sooner or later we are going to apprehend the culprits and they will be taken to justice," he added.

Some 20 U.S. states have initiated divestment efforts due to the ethnic and political conflict in Darfur, which international experts say has taken 200,000 lives and displaced 2.5 million people since rebels took up arms against the government in 2003. Khartoum says 9,000 people have died.

On Monday, a joint U.N.-African Union (AU) force took charge of peacekeeping in Darfur, replacing a struggling AU mission.

The plan is for it to comprise 20,000 soldiers and 6,000 police, but current numbers are only about a third of those levels. (Additional reporting by Inal Ersan in Dubai; Writing by Cynthia Johnston; Editing by Stephen Weeks)

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