Rebels from Sudan's Darfur region have attacked a city in a neighbouring state, taking their fight closer to Khartoum.
Fighters stormed Um Rawaba, in North Kordofan state, about 500km south of the capital, but denied accusations of pillaging.
Al Jazeera's Harriet Martin, reporting from Khartoum, said witnesses reported seeing rebels looting a market and several banks during the attack on Saturday.
Martin said the attack comes in the wake of increased fighting in South Darfur over the past few weeks, which has displaced thousands of people.
"This is a significant shift in the war in Sudan," Abdel Wahid Mohammed al-Nur, who heads a faction of Darfur's Sudan Liberation Army, told AFP.
"We are heading to Khartoum," he said. "This is not a joke."
The move marks the biggest push yet by a rebel alliance, that includes the SLA and the Justice and Equality Movement (JEM), and seeks to topple President Omar al-Bashir.
In 2011, JEM teamed up with the the SLM Abdul Wahid, and the Sudan People's Liberation Movement-North (SPLM-North), which took up arms in South Kordofan and Blue Nile states around the time of South Sudan's secession.
They formed the Sudanese Revolutionary Front (SRF), which says it fights to topple Bashir to secure a fairer share of government in a country dominated by three Arab tribes.
Fighting had previously been limited mainly to the remote regions of Darfur and South Kordofan and Blue Nile states, which border South Sudan
Martin said Saturday's attack was unusual because it was lay outside of these current areas of conflict.
Sudan started peace talks with the SPLM-North on Wednesday after a thaw in relations with South Sudan.
The government wants to broker a regional deal in South Kordofan and Blue Nile to end the conflict.
The potential deal not only poses a threat to the SRF, but could split the SPLM-N as some leaders remain loyal to the rebel alliance whereas others want to sign up to the local settlement.
Talks broke down on Saturday following disagreements over the issue of humanitarian access, but are expected to reconvene at a later date.
Martin said violence on the ground often increased when there are talks surrounding a potential ceasefire.
In a statement, the SPLM-N confirmed it had been involved in fighting in South Kordofan, but took no responsibility for clashes in the north.
'Topple the regime'
Sudan's army told state media on Saturday that it was still fighting rebels inside Um Rawaba, the state's second largest city.
Sudan Armed Forces (SAF) spokesman Sawarmi Khaled Saad said troops confronted the rebels after they reached Abu Kershola in the far north of South Kordofan.
Rebels then looted Kareem Allaha village before targeting Umm Rawaba town in North Kordofan, Saad said.
The army accused the rebels 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 the agricultural export product, gum arabic.
"Our forces are controlling parts of eastern North Kordofan and Um Rawaba," JEM spokesman Gibril Adam said.
"The goal of this attack is to weaken the government to realise our strategic plan to topple the regime."
'Rebels have fled southwards'
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-afflilated Sudanese Media Centre.
"The rebels have fled southwards," he said. "They were unable to stay in Um Rawaba."
JEM was one of two main rebel forces that took up arms against Sudan's government in 2003, demanding better representation for Darfur and accusing Khartoum of neglecting its development.
Khartoum mobilised armed groups to crush the uprising, unleashing a campaign that the US and activists describe as genocide.
Sudan's government denies the charge and accuses the Western media of exaggerating the conflict.