CAIRO, May 1 (Reuters) - The human rights group Amnesty International said late Friday that fighting was persisting in western Sudan despite a cease-fire between the government and rebels, and that time was running out to avert a disaster among civilians before the rainy season.

The Sudanese government and two main rebel factions in the west, the impoverished Darfur region, signed a truce on April 8 to allow aid to reach about one million people.

Relief officials say rains are expected to start in late May and could hinder the distribution of aid and medical supplies. Rebels took up arms against the government in February 2003 to push for a larger share of power and Sudan's resources.

Amnesty said, "Two time bombs are ticking in Sudan in a countdown to disaster: the approaching rainy season, which means that by June many areas may be cut off from food and medical supplies from outside; and the danger that a complete collapse of the cease-fire will lead to an escalation of violations."

Attacks on villages, indiscriminate and deliberate killings of civilians, rape and lootings were continuing, Amnesty said.

Monitors from the African Union designated to investigate cease-fire violations were not in place, it added.

An official from the Sudan Liberation Army, one of the two main rebel groups, said Thursday that Arab militias from Darfur had crossed about six miles into Chad and attacked refugees and local villages.

Rebels and others accuse the government of arming the Arab militias, known locally as janjaweed, to loot and burn African villages. The government in Khartoum calls the militias outlaws.

"Unless the international community puts maximum pressure to ensure that the government militia are disarmed and removed from the region, the conflict will worsen and spread," Amnesty said.

Amnesty said most villages in Darfur had been destroyed.

United Nations officials have said the situation in Darfur is one of the world's worst crises, with more than 110,000 refugees camped in Chad.

The United Nations High Commissioner for Refugees said in a statement on Friday that its emergency relocation operation in eastern Chad had so far moved 45,000 Sudanese refugees away from the insecure Chad-Sudan border. It said it hoped to move at least 60,000 by the start of the rainy season.