The United States says "regional extremists" may be targeting Air Uganda flights between Southern Sudan and Uganda.
A warning posted Friday on the Web site of the U.S. Embassy in Khartoum says there is a "potential threat" on the flights between Juba, Sudan and Kampala, Uganda. Juba is the capital of semi-autonomous Southern Sudan.
The embassy did not name the potential attackers but said the threat is of "sufficient seriousness," and that air travelers should "maintain vigilance at all times."
The U.S. has long warned its citizens against traveling to Sudan.
Sudan is due to hold its first multiparty elections in more than two decades in April. However, tensions are running high between the north and south over a disputed census, border issues, and a scheduled 2011 referendum on independence by the south.
April's vote is a key part of the peace agreement between Sudan's northern and southern regions that ended a 21-year civil war in 2005.
Some information for this report was provided by AP and Reuters.