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 US airliner blamed on a Nigerian man who US officials believe was trained by al-Qa'ida in Yemen.
The United States 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 yesterday on their website 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.
It added 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 the potential attackers but has said in the past that some groups were active in Sudan.
Somali al Shabaab rebels in October threatened to strike Kampala and Bujumbura in revenge for rocket attacks by peacekeepers from those countries that killed at least 30 people in Mogadishu.
Washington says al Shabaab has close ties with al-Qa'ida.
Uganda and Burundi both have about 2,500 peacekeepers in the Somali capital for the African Union's AMISOM mission.
Security at Juba airport is lax. A Reuters witness said the only scanner in the airport was not working last week and security staff do not go beyond hand searches of luggage.
Anne Itto, a senior member of southern Sudan's dominant Sudan People's Liberation Movement (SPLM), told reporters she was sure the region's security services were aware of the threat "and they will take care of it".
Air Uganda runs daily flights between Juba and Uganda's Entebbe airport.
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.
US officials have acknowledged Sudan has been cooperative in sharing intelligence on militant groups since the September 11 attacks in 2001. The Sudanese government has repeatedly denied al-Qa'ida has an active presence in Sudan.