Rebels in Sudan's Blue Nile state on Wednesday said they seized an airstrip outside a key town near the Ethiopian border in some of the heaviest fighting in months.
The Sudan People's Liberation Movement-North (SPLM-N) said they were just a few kilometres (miles) from El Kurmuk, the major town in southeastern Blue Nile state.
"It's very close and we are controlling the Kurmuk airstrip," which is west of the town, rebel spokesman Arnu Ngutulu Lodi told AFP.
"This is a very serious one," he said of the battle.
The army denied there was fighting on the outskirts of Kurmuk but said its forces had withdrawn from an area southwest of the town which, two days earlier, it announced it had "liberated."
Access to Blue Nile is restricted, making verification of claims difficult.
Khartoum accuses South Sudan, which became independent in 2011, of backing rebels in South Kordofan and Blue Nile, but the South denies such support.
Concern has been rising over tensions between Khartoum and Juba after they failed to implement agreements which they hailed in September as ending conflict.
The pacts included a demilitarised border zone.
France's foreign ministry on Tuesday expressed concern "about the continued fighting" between Sudanese forces and the SPLM-N in Blue Nile and another state, South Kordofan.
The United States last Friday said it was "deeply concerned" by reports of clashes between Sudan and South Sudan in disputed border areas between Blue Nile and South Sudan's Upper Nile state.
Yasir Arman, secretary general of the SPLM-N, in a statement on Sunday accused the armed forces of launching "a military dry season campaign" in an area about 20 kilometres (12 miles) southwest of Blue Nile's El Kurmuk town.
He said there had been "heavy aerial bombardment from Sudan's air force", prompting 8,000 people to flee towards the borders of South Sudan and Ethiopia.
Figures from the humanitarian wing of the rebels along with data from the government's aid agency indicate more than one million people have been affected by unrest in Blue Nile and South Kordofan states.