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

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

The US stepped up security screenings of passengers travelling from or through Sudan and 13 other countries after the failed attack. US embassy staff in Khartoum published a warning late on Friday of “a potential threat against commercial aviation transiting between Juba [southern Sudan’s capital] and Kampala, Uganda”.

“The US 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.

The statement said it was not clear whether the group had the ability to mount an attack but warned air passengers to be alert. The US embassy did not name potential attackers but has said in the past that some groups were active in Sudan.

In October, Somalia’s al-Shabaab rebels threatened to strike Kampala and Bujumbura, the capital city of Burundi, in revenge for rocket attacks by peacekeepers from Uganda and Burundi, which killed at least 30 people in Mogadishu, Somalia’s capital. Washington says al-Shabaab has close ties with al-Qaeda. Uganda and Burundi both have about 2500 peacekeepers in Mogadishu.

Security at Sudan’s Juba airport is lax. An eyewitness reporter confirmed that the only scanner in the airport was not working last week and said that security staff do not go beyond hand-searches of luggage.

Anne Itto, a senior member of southern Sudan’s People’s Liberation Movement, said she was sure the region’s security services were aware of the threat “and they will take care of it”.

Sudan, which hosted al-Qaeda chief Osama bin Laden in the 1990s before expelling him, has been on a US list of state sponsors of terrorism since 1993.