Insurgent groups said Sudanese forces launched "very heavy" ground and air attacks on rebel positions and villages in North Darfur on Saturday, in the latest of a string of reported assaults.
The U.S. special envoy to Sudan, Richard Williamson, sharply condemned the Sudanese government's "pattern of violence," saying one of Saturday's attacks appeared to have targeted Minni Arcua Minnawi, a former rebel leader who signed the 2006 Darfur peace accord.
A spokesman for Sudan's Armed Forces told state media soldiers had deployed in the area to protect aid convoys from armed bandits but made no mention of any fighting, saying they had not met any resistance.
The accusations come at a particularly sensitive time as Khartoum steps up diplomatic efforts to block moves by the International Criminal Court to indict Sudan's president for genocide and other alleged crimes in Darfur.
More than five years of conflict there have killed 200,000 people and driven more than 2.5 million from their homes, say international experts. Khartoum says 10,000 have died.
"The Antonovs (military aircraft) have been bombing everywhere," said Ahmed Kubur Jibril, from the rebel Sudan Liberation Army's Unity (SLA-Unity) faction.
"Sometimes they have been bombing SLA positions. Sometimes they have been bombing villages and civilian areas. There were soldiers in many vehicles," he said by satellite phone from Darfur.
Jibril said the attacks on the villages of Tarny, Khazan Tungur and surrounding settlements southwest of the town of Tawila, were part of a week-long offensive on insurgent positions in North Darfur. Fighters have told him of civilian deaths and rebel injuries, but there were no definite figures.
"There has been some very heavy fighting," said Ibrahim al-Helwu, from another branch of the SLA.
In a telephone interview with Reuters, Williamson said the recent attacks appeared to be part of a "pattern of violence under the guise of a law-and-order campaign" by Khartoum.
He said the attacks had been going on for nearly a month.
The U.S. envoy said Saturday's attack in North Darfur state was especially worrying if Minnawi, now an adviser to Sudanese President Omar Hassan al-Bashir, was targeted, given the key role he plays in the Darfur peace process.
"This is very disturbing," Williamson said, adding that he had spoken with Minnawi earlier on Saturday. "The government should honor the ceasefire. This descent into violence causes grave concern about the humanitarian situation."
"The U.S. government condemns this. It has to stop."
Other rebel leaders have accused Khartoum of launching the attacks to secure key transport routes and grab territory ahead of possible negotiations led by the Arab League and the new joint U.N./African Union mediator for Darfur, Djibril Bassole.
A spokesman for the U.N./AU peacekeepers in Darfur said he was checking the reports of fresh fighting. The undermanned and ill-equipped mission earlier said it could not confirm reports from rebels in North Darfur as it did not have bases in the area or enough helicopters to fly over the locations.
But this week it added that officers had seen large-scale movements of Sudanese army equipment, suggesting "intense military activity" was taking place.
Khalil Ibrahim, leader of Darfur's powerful rebel Justice and Equality Movement, gave a guarded welcome to an offer from the Arab League to form a ministerial committee to find a solution to the Darfur conflict.