Sudan's north-south civil war ended in 2005 with a peace deal that shared wealth and power, enshrined democratic transformation and allowed southerners to vote in a Jan. 9 plebiscite which most expect to result in secession.
Sudan's separate rebellion in Darfur — which is part of the north — began in early 2003 and numerous truces have failed to stem violence there. The International Criminal Court has indicted President Omar Bashir for war crimes there.
"If you are accommodating these forces in the south, you are supplying these forces with weapons, logistics, petrol and cars ... we think that this is a declaration of war against the north of the country," Mandour Mahdi, a senior official from the northern National Congress Party, told Reuters.
Abdel Wahed Mohamed El-Nur, leader of the Sudan Liberation Army (SLA), one of Darfur's main rebel groups, will visit the southern capital Juba in the coming days, his spokesman said, and other rebel leaders have visited or reside there.
Mahdi said the Darfur rebel Justice and Equality Movement (JEM) had moved its forces to the south to receive training. Earlier this month the north accidentally bombed the south while fighting the JEM near the north-south border.
South Sudan's army was not immediately available for comment but denies aiding rebels from Darfur. The south has in the past hosted unity talks between the fractious rebels to try to help move the Darfur peace process forward. If the south separates, Darfur will remain part of the north.
The dispute marks a low point in north-south relations which have been tense in the build up to the plebiscite. Talks on resolving the status of the disputed Abyei region are deadlocked and little progress has been made on defining citizenship, the border or other post-referendum issues.
Mahdi said the south's support for Darfur rebels was affecting talks covering security arrangements after the referendum. "They should expel these forces out of south Sudan ... overall I hope that we reach a settlement of this issue so as not to affect the referendum," he said.
