A rebel group in Sudan's western Darfur region says the government is responsible for an upsurge in fighting there in violation of a ceasefire. But U.S. and U.N. officials say the rebels started the latest violence with an attack that killed more than 20 policemen.

A spokesman for the Sudan Liberation Movement says his group remains committed to a peaceful settlement of the Darfur conflict, despite the breakdown of the cease-fire.

The Sudan Liberation Movement foreign affairs spokesman, Ahmed Abdel Shafie Yagoub, has told reporters in Nairobi the Khartoum government is to blame for the renewed fighting.

"SLM is concerned about the recent deterioration in the security situation in Darfur, which is being deliberately escalated by the government in Sudan," said Ahmed Abdel Shafie Yagoub.

The State Department and the chief U.N. envoy to Sudan say the rebels broke the cease-fire with an attack Monday on Sudanese policemen in the town of Tawillah, about 50 kilometers west of El-Fasher, the capital of North Darfur state.

The U.N. representative, Jan Pronk, says the fighting has placed the lives of tens of thousands of displaced people at risk by disrupting humanitarian aid flows.

But the SLM. spokesman, Mr. Yagoub, says his group is only defending itself from attacks by the Sudanese military and a Khartoum-backed Arab militia called the Janjaweed. He asserts that international cease-fire monitors have been infiltrated by Khartoum government agents and are giving a distorted picture of the conflict.

"They report both of us, the Sudan Liberation Movement and the government as violating equally the cease-fire, whereas that is not true," he said. "All these violations took place in our positions. So they [the military and militias] are coming, they are attacking and we are defending ourselves."

The rebel spokesman is calling for more and better-equipped African Union peacekeepers to come to Darfur, where 22 months of fighting has displaced about 1.5 million people, with an estimated 70,000 killed.