Fighters from the Sudan Liberation Army (SLA) in Darfur, in the west of Sudan, have said that the north's Sudan Armed Forces (SAF) has attacked them with war planes and military vehicles.
The SLA said the attacks resulted in 27 deaths, 40 injuries and the displacement of thousands of people.
The reports come as satellite images appeared to show the north Sudanese military massing in the southern border state of Kordofan, a monitoring group said.
Southern Kordofan, on the ill-defined border with the south, is among several flashpoints as Sudan's south prepares to secede on July 9, a move analysts say could embolden fighters in other parts of the country.
The SAF confirmed it had clashed with fighters in Darfur on Sunday in the mountainous Jabel Marra region, but said it had not used aircraft and the fighting had not displaced civilians.
Ibrahim al-Helwu, a spokesperson for SLA, the said the violence began when government troops advanced from the Darfur settlements of Kas and Nyala.
He said 27 people, including 19 civilians, were killed and about 40 wounded after an assault by land troops as well as Antonov and MiG aircraft.
"From the morning, the government started to attack," Helwu said, speaking by phone from Paris. "More than 10,000 civilians are displaced from this area".
The northern army said the fighting in the Jabel Marra region had caused an unconfirmed number of casualties on both sides and that no civilians had been harmed.
Violence in Darfur, where mostly non-Arab fighters are fighting government troops and largely Arab armed groups, has fallen from its peak in 2003 and 2004, but a surge in attacks since December has forced tens of thousands to flee.
The International Criminal Court has issued an arrest warrant for Omar Hassan al-Bashir, the Sudanese president, on charges of masterminding genocide and war crimes in Darfur.
Khartoum refuses to recognise the court. The UN says as many as 300,000 people have died during the conflict in Darfur. Khartoum puts the death toll at 10,000.
The Satellite Sentinel Project (SSP), which monitors Sudan, said on Sunday that new imagery from Friday showed that the SAF controlled Kadugli, the capital of Southern Kordofan, and that thousands of civilians had been displaced.
The images "show a massing of SAF artillery, light vehicles and heavy transports of the kinds used to carry tanks, troops, and munitions," it said.
Fighting between the SAF and southern-aligned groups has spread across the north-run oil state since June 5.
Tens of thousands of people have fled the area, according to the UN.
Al-Sawarmi Khaled, an SAF spokesperson, said: "We are now fighting harder to control all the area in Southern Kordofan."
The SSP, Set up last year by Hollywood actor George Clooney and other activists, says it seeks to head off renewed fighting and atrocities in Sudan by publishing commercial satellite images collated and analysed with the help of a UN agency.
Southern Kordofan, the main oil-producing state that will be left in the north after the south secedes, is home to thousands of fighters who fought against Khartoum during the last civil war, many of them from the Nuba mountains region.
Officials with the south's dominant Sudan People's Liberation Movement (SPLM) say clashes started when the north tried to disarm fighters there.
Northern officials blame southern-aligned groups for provoking the fighting after an official from the north's ruling National Congress Party was named winner of a state gubernatorial election last month.
In Khartoum, 16 political activists said they were detained for about four hours on Sunday after attempting to stage a demonstration against violence in Southern Kordofan.
The south voted to secede in a January referendum promised in a 2005 peace deal that ended decades of civil war.
At least seven different armed groups are also fighting the southern Juba government, according to the UN.