By Andrew Heavens

KHARTOUM, Oct 11 (Reuters) - The United States said on Saturday a group called "Al Qaeda in the Land of the Two Niles" has threatened its citizens and allies in Sudan, adding to fears of a growing extremist presence in the country.

Embassy officials also said they had warned staff against using a Khartoum cafe popular with Westerners and rich Sudanese.

The Sudanese government has repeatedly denied al Qaeda has an active presence in Sudan.

A message on the U.S. embassy website said the group had issued a statement which referred to the murder of U.S. aid official John Granville and his driver on Jan. 1.

The group said its "jihad and fight against the United States of America and its allies of crusaders and apostates will continue," according to the embassy.

Another U.S. embassy message said it had warned staff not to use the Ozone cafe in an affluent Khartoum district as it was "particularly vulnerable".

Ozone is an open air cafe in the centre of a busy roundabout surrounded by houses and businesses. Staff and customers are clearly visible from the road and buildings.

An embassy spokeswoman said she could not comment on whether the Ozone warning was linked to the Al Qaeda threat.

There have been growing fears that extremist groups have set up operations in Khartoum.

In August 2007, Sudanese security services said they had broken up a plot to attack the French, British, U.S. and U.N. diplomatic missions in Khartoum.

The group was discovered in a Khartoum house after explosives went off by accident, foreign sources said.

Five men are currently appearing in court in Khartoum charged with the murder of John Granville and his driver Abdelrahman Abbas Rahama 1. All five deny the charge.

Prosecutors said the group that killed Granville and his driver targeted Americans it thought were trying to "Cristianise" the predominantly Muslim nation.

Earlier this year, al Qaeda graffiti also started appearing on walls in the capital.

Sudan, which hosted al Qaeda chief Osama bin Laden in the 1990s, has been on a U.S. list of state sponsors of terrorism since 1993.

(Editing by Angus MacSwan)

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