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KHARTOUM (Reuters) - The United States has warned that "regional extremists" were planning an attack on Air Uganda flights between southern Sudan and Kampala.

Uganda's army said it was aware of the threat and was taking precautions. "We're a constant target of these extremists and are always alert, so there is no cause for alarm," Uganda's army spokesman, Major Felix Kulayigye, told Reuters.

The warning came amid heightened tensions following the botched Christmas Day bombing attempt on a Detroit-bound U.S. airliner blamed on a Nigerian man who U.S. officials believe was trained by al Qaeda in Yemen.

The United States stepped up security screenings of passengers travelling from or through Sudan and 13 other countries after the failed attack.

U.S. embassy staff in Khartoum published a warning late on Friday on their website of "a potential threat against commercial aviation transiting between Juba (southern Sudan's capital) and Kampala, Uganda".

"The U.S. Embassy has received information indicating a desire by regional extremists to conduct a deadly attack onboard Air Uganda aircraft on this route," the embassy statement read.

It added it was not clear whether the group had the ability to mount an attack but warned air passengers to be alert.

The U.S. Embassy did not name the potential attackers but has said in the past that some groups were active in Sudan.