Sudanese troops have clashed with rebels in South Kordofan after months of calm in the state, the two sides said on Tuesday, with both giving differing accounts of the fighting.

President Omar al-Bashir's forces have been battling the Sudan People's Liberation Movement-North (SPLM-N) in the South Kordofan and Blue Nile areas since 2011 but in recent months the two states had been quiet compared with previous years.

"The SPLM-N confirms that there are ongoing battles" around the Karkaraya and Agab areas of South Kordofan, rebel spokesman Arnu Lodi said in a statement late Tuesday.

Lodi said the rebels were in "full control of the situation" and were laying siege to government positions, having suffered no casualties of their own in the operations.

The military confirmed there had been heavy fighting around Karkaraya but said its soldiers had repulsed an SPLM-N column that attacked from the nearby rebel-held town of Um Serdiba.

Early on Tuesday, "a large force of rebels came out of the town of Um Serdiba and attacked the area of Karkaraya that we recaptured" the day before, army spokesman Brigadier Ahmed Khalifa al-Shami told AFP by phone.

The military said it had destroyed a number of SPLM-N vehicles and killed 55 rebels, suffering no casualties of its own.

Khartoum limits press access to the war-hit border regions, making it nearly impossible to verify the often contradicting reports from the military and the SPLM-N.

The rebels in the two southern states mounted an insurgency against Bashir's Arab-dominated Khartoum government in 2011, and neither side has decisively gained the upper hand in the fighting.

Bashir announced a ceasefire in South Kordofan, Blue Nile and the western Darfur region -- the site of a separate insurgency -- late last year and extended it by a month at the beginning of the year.

Despite both sides accusing the other of continuing attacks, South Kordofan had been relatively peaceful until this week's fighting.

A government delegation travelled to Addis Ababa for an African Union-mediated meeting with the rebels and opposition parties, although the talks ended on March 21 without a conclusive result.

There has also been heavy fighting this year in the Jebel Marra area of Darfur, with tens of thousands of civilians reported to have been forced to flee their homes in the clashes that started in January.