A large force of Sudanese soldiers backed by janjaweed militias is sweeping through the towns and villages of North Darfur in spite of a cease-fire, international observers and rebels in Darfur said Sunday. At least four civilians were killed near the northern town of Birmaza on Sunday, Youssouf Mussabal, a rebel leader in the area, said. 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 the renewed government offensive began earlier this week, and that two civilians were killed in Friday raids. "Seven villages were also looted and burnt to the ground around Birmaza today," Jar al-Naby, the rebel commander, said in a phone interview from Darfur. Hundreds of heads of cattle have been rounded and brought back to the Sudanese army headquarters in the North Darfur town of Mellit, he added. The African Union peacekeeping force in Darfur said it received reports the Sudanese air force twice bombed Birmaza this week. The attacks were jointly conducted by the army and what the AU described as "armed militia" groups. The offensive has had a "heavy toll on the civilian population," the AU said in a statement issued Saturday. "These attacks are a flagrant violation" of the Darfur Peace Agreement, signed in May by the government and one rebel group, the statement said. According to a senior United Nations official in Sudan, five children were killed in a joint janjaweed and government offensive to the east of the same zone on Friday. Another UN official in North Darfur said 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. Both officials spoke on condition of anonymity because of the sensitivity of the issue. Al-Naby, the rebel field commander, said his faction had waged a large battle against combined army and janjaweed forces on Saturday near Saiyah, some 25 kilometers north of Mellit. He said the government force attacked about 100 pickup trucks, the common means of transport and warfare in the region. He said six rebels had been killed, including a field commander named Sadiq Abou, and that eight were seriously injured. Al-Naby sated that government forces suffered dozens of casualties, including a military commander, and that rebels destroyed nine pickups, capturing another 21. The Sudanese military was not immediately available for comment Sunday, and the rebels' battle claim could not be independently verified. In recent weeks, several foreign journalists have been unable to obtain permits from the Sudanese government to enter Darfur. On Saturday, UN 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." "The statements (Egeland) made are a pure political act that seeks to boost (UN Security Council) Resolution 1706," Hassabu Mohammed Abdallah, the humanitarian aid commissioner at the ministry, said. The Security Council passed resolution 1706 in August to send over 20,000 peacekeepers in Darfur to replace the 7,000 AU force that has had little success in stopping the bloodshed. Sudan's government rejected the resolution, but appeared to accept this week a compromise by which a mixed UN-AU force would take over. The exact nature of this hybrid mission and its scheduling were still being debated Sunday. 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. Khartoum denies having unleashed the janjaweed militias as part of its counterinsurgency response, and says these Arab tribal fighters are bandits it does not control. The army denied earlier this week 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, amid reports that children had been rounded and burned alive by militias.