U.S. President Barack Obama has written to African leaders to reiterate that Sudan is a foreign policy priority for his administration, both during the lead-up to the January 9 referendum on independence for the south, and in response to the continuing crisis in Darfur.

"This is yet another element of an ongoing aggressive diplomatic effort with the parties in Sudan and with its neighbors reflecting our intense interest in having a successful referendum," White House National Security Council spokesman Mike Hammer said in a statement.

"We believe that an on-time referendum is the best means of preventing the resumption of a full-scale war between northern and southern Sudan," Hammer said.

"Over the past four months, the Administration has redoubled our efforts to support referendum preparations and peace negotiations between the two parties."

The letter from Obama was sent to Egypt, Chad, Uganda, Kenya, Libya, Ethiopia, South Africa, Nigeria and Rwanda, as well as the African Union, a senior White House official told AllAfrica.

Egypt's Hosni Mubarak and Libya's Muammar al-Gaddafi are scheduled to visit Khartoum on Tuesday for discussions with Sudanese President Omar al-Bashir and southern Sudanese leader Salva Kiir aimed at resolving outstanding issues relating to the referendum and the post-referendum period.