The Sudanese army says it has sent more troops to the sensitive oil-rich South Kordofan state.
The army told state media that it had information that a Darfur rebel group planned to attack the area.
The main party in the south says the military build-up is a violation of a 2005 peace deal that ended civil war.
There has been speculation in Khartoum for weeks that large numbers of Darfur rebels had crossed into Kordofan, followed by northern soldiers.
Sudan’s armed forces have now confirmed the military build-up.
An army spokesman said the aim was to prevent any effort by the Darfur rebel Justice and Equality Movement (Jem) to extend its activities into South Kordofan.
Many observers have been concerned about the Darfur conflict spilling over into neighbouring states.
The armed forces have been on alert since Jem launched an unprecedented attack on the Sudanese capital in May, taking politicians and Sudan’s security forces by surprise.
The movement of northern soldiers into Kordofan has stirred tensions with the north’s power-sharing partner, the Sudan People’s Liberation Movement (SPLM), the main party in the south.
An SPLM spokesman said the North’s decision to send six extra battalions to Kordofan was a violation of a peace deal signed in 2005 between the North and South ending a two-decade civil war.
He said the group not been informed and was worried that the move could spark conflict with its troops on the ground.