KHARTOUM, Sudan -- A large force of Sudanese soldiers backed by allied janjaweed militiamen is sweeping through North Darfur, killing civilians and looting and burning villages in violation of a cease-fire agreement, international observers and rebels said Sunday.

At least four civilians were killed near the northern town of Birmaza on Sunday, said Youssouf Mussabal, a rebel leader in the area. Some 200 pro-government janjaweed fighters riding camels had moved into the zone, backed by mobile army units and the Sudanese air force, he added.

"The janjaweed are still in the town. We're worried for the population," Mussabal told The Associated Press by telephone from North Darfur.

Another rebel field commander from a separate faction said seven villages were looted and burned to the ground around Birmaza on Sunday. Jar al-Naby said the renewed government offensive began earlier this week, and that two civilians were killed in Friday raids.

Hundreds of heads of cattle have also been rounded and brought back to the Sudanese army headquarters in the North Darfur town of Mellit, he said by phone from Darfur.

A senior U.N. official in North Darfur said Sunday international observers were receiving daily reports of raids and casualties throughout this vast area of semi-desert pastureland north of the regional capital of El Fasher.

"The campaign is ongoing, and we are being given very limited access to investigate or treat casualties," the official said on the phone from North Darfur. The official spoke on condition of anonymity because of the sensitivity of the issue.

The African Union peacekeeping force in Darfur said in a statement Saturday it had received reports that the Sudanese air force twice bombed Birmaza this week. The attacks, conducted jointly with armed militia groups, took a "heavy toll on the civilian population," the AU said.

The statement said the attacks were a "flagrant violation" of the Darfur Peace Agreement, signed in May by the government and one rebel group.

The Sudanese military was not immediately available for comment Sunday. Earlier this week, however, the army denied it was conducting an offensive in North Darfur, and categorically denied it was using the janjaweed to attack civilians.

"We as army cannot engage in any such attacks or make use of militias to attack our own citizens," the office of the army spokesman said Wednesday.

More than 200,000 people have been killed and 2.5 million displaced since fighting began in Darfur three years ago, when rebels stemming from ethnic African tribes rose up against the Arab-led central government.

Al-Naby, the rebel field commander, said his faction had waged a large battle against combined army and janjaweed forces on Saturday near Saiyah, about 15 miles north of Mellit. He said six rebels had been killed, including a field commander, and that eight were seriously injured.

Al-Naby said government forces suffered dozens of casualties, including a military commander, and that rebels destroyed nine pickups and captured another 21.

The rebels' battle claim could not be independently verified. In recent weeks, foreign journalists have been unable to obtain permits from the Sudanese government to enter Darfur.

On Saturday, U.N. chief of humanitarian affairs Jan Egeland said he had been barred by Sudanese authorities from visiting several of the areas where fighting is now reported.

"The government is arming Arab militias more than ever before," Egeland said in an AP interview. He warned that the crisis "still has the potential of becoming infinitely worse."

The Sudanese Ministry of Humanitarian Affairs said Sunday that Egeland was spreading "lies and false accusations against the Sudan."

Hassabu Mohammed Abdallah, the humanitarian aid commissioner at the ministry, said Egeland's remarks were "a pure political act" seeking to boost the U.N. Security Council resolution that calls for the world body to send more than 20,000 peacekeepers to Darfur to replace the 7,000-strong AU force that has had little success in stopping the bloodshed.

Sudan's government has rejected the resolution, but this week appeared to accept a compromise in which a mixed U.N.-AU force would take over. The exact nature of this hybrid mission and the timing of the deployment were still being debated Sunday.