Sudan's army has killed a key rebel leader from the western Darfur region along with 30 of his troops, officials said, three days after the Justice and Equality Movement said it had begun advancing on the capital Khartoum.
The government on Sunday was touting the death of Khalil Ibrahim as a key victory over the JEM, which had rejected a peace deal.
"The Sudanese army announce that they killed Khalil Ibrahim in fighting today west of Wad Banda in North Kordofan," the official Sudan News Agency reported.
The army said Ibrahim was wounded in fighting on Thursday and died of his wounds on Saturday. Abdullah Ali Massar, the country's information minister, said at a news conference later that the battle was continuing.
JEM, however, said its leader died in an air strike and not a "clash" with government forces.
In a statement posted on the group's website, it said that Ibrahim was killed early on Friday when his camp was attacked by a plane, and added that "the martyr's blood will not be wasted, and those who killed him will pay the price more than double".
A UK-based rebel spokesman said Ibrahim was killed by a fighter plane directed by "a spy," and denied there was a battle between rebel forces and government troops. "When Khalil was attacked he was in his camp," the spokesman said.
Ibrahim had headed the JEM, the most heavily armed group in the Darfur region, which the government had accused of being behind earlier attacks.
On Saturday, the Sudanese army said the JEM attacked three areas in North Kordofan state - adjacent to Darfur - killing an unspecified number of civilians.
Colonel Sawarmi Khalid Saad, an army spokesman, was quoted on Sudan's military website as saying the attacks targeted areas where security forces were not present, targeting local leaders and looting their property in the Umm-Gozain, Goz Abyadh and Aramal areas.
Road to Khartoum
The death of Ibrahim could represent a serious blow to the rebel group, although tightly restricted access to Darfur and Sudan's other conflict zones makes it almost impossible to accurately gauge the strength and unity of rebel groups.
The armed forces spokesman said Ibrahim and other leaders had been trying to enter South Sudan, which seceded from Sudan in July under a 2005 peace deal that ended a separate, decades-long civil war.
A JEM spokesman said on Thursday the group had reached En Nahud, about 120km east of Darfur in North Kordofan, on a mission to topple President Omar al-Bashir.
"Ibrahim was of the Zaghawa tribe but his movement includes other tribes. He's even been supported by Islamists in Sudan," Al Jazeera's Mohamed Vall reported from Khartoum.
Ibrahim was believed to have recently been living in exile in Libya, where he enjoyed the support of Muammar Gaddafi until the death of the Libyan dictator at the hands of the country's revolutionaries in October.
"But now that [Ibrahim] is gone, we don't know if other leaders from JEM can keep support without help from other countries," our correspondent said, describing the loss of main JEM allies with the fall of Gaddafi and abandonment by Chad's leader, Idriss Deby.
In the past, JEM has launched attacks in Kordofan, about 700km west of Khartoum. They were also behind a 2008 attack in the capital.
In that incident, more than 222 people were killed when JEM guerrillas drove about 1,000km across the desert to Omdurman, just across the River Nile from the presidential palace.
Government troops repulsed them after heavy clashes and later sentenced dozens of captured rebels to death.
In July, the government signed the Doha Document for Peace in Darfur with the Liberation and Justice Movement, an alliance of splinter rebel factions.
The agreement, reached in Qatar and backed by the African Union, was seen as a basis for a cease-fire, power sharing, equal distribution of wealth and compensation for displaced people.
JEM and factions of the Sudan Liberation Army headed by Minni Minnawi and Abdelwahid Nur, did not sign the deal.
Instead, along with the SPLM-North rebels, they last month ratified documents forming the new Sudanese Revolutionary Front dedicated to "popular uprising and armed rebellion" against the National Congress Party regime in Khartoum.
According to the UN at least 300,000 people have been killed in Darfur since 2003 when fighting broke out between non-Arab rebels and the Arab-dominated Khartoum regime. The government puts the death toll at 10,000.
UN officials say 1.9 million people are internally displaced and still living in camps in Darfur, with about 80,000 newly displaced by fighting this year.
Six people, including President Bashir, are being sought or are before the Hague-based International Criminal Court for crimes allegedly committed in the Darfur region.