Rebels from Sudan's Darfur region attacked a city in a neighboring state on Saturday, taking their fight closer to the capital Khartoum, witnesses said.

The attack marks the biggest push of a rebel alliance that seeks to topple President Omar Hassan al-Bashir. Fighting had hitherto been limited mainly to the remote regions of Darfur and South Kordofan and Blue Nile states, which border South Sudan.

The Justice and Equality Movement (JEM) - which launched an unprecedented assault on Khartoum in 2008 - said insurgents stormed the city of Um Rawaba in North Kordofan state, around 500 km south of the capital.

JEM did not say whether it would try to advance further.

Sudan's army told state media it was still fighting rebels inside Um Rawaba, 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, an important market for a Sudanese agricultural export product, gum arabic, and looted a market and several commercial banks, residents told Reuters. A JEM spokesman denied any pillaging by rebels.

"Our forces are controlling parts of eastern North Kordofan and Um Rawaba," spokesman Gibril Adam said. "The goal of this attack is to weaken the government to realize our strategic plan to topple the regime."

The government later reopened the road between Khartoum and the North Kordofan state capital El-Obeid, which had been blocked by fighting, state governor Mutassim Mirghani Zaki Uddi told the state-affiliated Sudanese Media Center (SMC).

"The rebels have fled southwards," he said. "They were unable to stay in Um Rawaba."

Events outside Khartoum are difficult to verify in the vast African country. Um Rawaba is a two-hour drive from Kosti, Sudan's biggest Nile river port.-Reuters