Sudanese President Omar al-Bashir issued the proclamation Sunday for cities in the eastern half of the border region and then reportedly launched a series of airstrikes and helicopter attacks against South Sudanese troops.
CNN said reports from journalists inside South Sudan indicated troops there had little in the way of anti-aircraft weapons to defend themselves against the choppers and MiG fighters.
Casualties Sunday appeared to be light; however, the border region has been steeped in violence over the years. South Sudan became an autonomous region in 2005 in a deal to end a bloody civil war and became an independent nation last July.
The two sides most recently have been skirmishing over the oil-producing region of Heglig. South Sudan earlier this month moved into Heglig but then withdrew after Sudan protested to the United Nations and the African Union.