Rebels from Sudan's Darfur region attacked a city in a neighbouring state on Saturday, taking their fight closer to the capital, witnesses said.
The Justice and Equality Movement (JEM) - which launched an unprecedented assault on Khartoum in 2008 - said it and other fighters attacked Um Rawaba in North Kordofan state, around 500km (300 miles) south of the capital.
The group did not say whether it planned to push further.
Sudan's army told state media it was still fighting rebels inside the state's second largest city. It accused the insurgents of destroying a power plant, petrol stations and a telecommunications tower.
"Battles are still ongoing," army spokesman al-Sawarmi Khalid told state news agency SUNA.
Armed men in 20 trucks drove into Um Rawaba and looted a market and several commercial banks, residents told Reuters.
"Our forces are controlling parts of eastern North Kordofan and Um Rawaba," JEM spokesman Gibril Adam said, adding that the fighting had blocked the road between Khartoum and El-Obeid, the capital of North Kordofan state.
"The goal of this attack is to weaken the government to realise our strategic plan to topple the regime," said Adam, who denied the rebels had looted any property in the city.
JEM forces drove across hundreds of miles of desert to attack the Khartoum suburb of Omdurman in May 2008 and were stopped just short of the presidential palace and army headquarters.
The group was one of two main rebel forces that took up arms against Sudan's government in 2003, demanding better representation for the remote western region of Darfur and accusing Khartoum of neglecting its development.
Khartoum mobilised militias to crush the uprising, unleashing a campaign that Washington and activists described as genocide. Sudan's government denies the charge and accuses the Western media of exaggerating the conflict.
JEM, which has fought the government in neighbouring states before, says it also wants fairer government across all of Sudan.
It is part of an alliance with insurgents in South Kordofan and Blue Nile states bordering South Sudan, and has vowed to overthrow President Omar Hassan al-Bashir.