GENEVA — Up to 12,000 refugees fled Sudan’s Darfur region to neighboring Chad over the weekend following air strikes by the Sudanese military and thousands more may be coming, the U.N. refugee agency said yesterday.

The agency was bringing emergency assistance to the Chad border where the Darfur refugees were giving detailed descriptions of air attacks Friday on three West Darfur towns.

The refugees are “destitute and terrified,” a spokeswoman for the U.N. High Commissioner for Refugees headquartered in Geneva, Helene Caux, said. “They told of their villages being looted and burned, and encircled by militia.” Most of the new refugees in Chad are men, and they told the United Nations that thousands of women and children are on their way, Ms. Caux added.

U.N. officials say the worsening situation in Darfur has been exacerbated by a recent rebel attack on the capital of neighboring Chad. Chad has accused Sudan’s President al-Bashir of backing those rebels in a bid to prevent deployment of a European peacekeeping force in the Chad-Sudan border region where some 400,000 refugees are living.

Sudan’s Arab-dominated government has been accused of unleashing more attacks by its allied Janjaweed militias, which are accused of committing the worst atrocities against Darfur’s ethnic African communities. At least 200,000 people have been killed and 2.5 million displaced since the violence began five years ago.

On Friday, Sudanese helicopter gunships and fixed-wing aircraft bombed the towns of Sirba, Sileia, and Abu Suruj while striking at rebel forces, which have been trying to consolidate their positions in West Darfur. Several U.N. resolutions ban military flights over the region, but the Sudanese military has regularly ignored them. The Sudanese army said its attacks forced rebels to retreat into neighboring Chad, a provocative accusation at a time of escalating tension between the two countries. Both nations accuse each other of hosting hostile rebel groups, allegations that became even more sensitive after Chadian rebels attacked Chad’s capital last weekend. Darfur rebels have denied any of their fighters were in the towns attacked by the government Friday, and said some 200 people were killed.

Ms. Caux said the refugees are reporting that their villages were also attacked by men on horses and camels, a description similar to those provided of earlier incidents involving the Janjaweed.

Secretary-General Ban on Saturday strongly condemned the attacks on West Darfur and demanded that all parties adhere to international humanitarian law, which prohibits military attacks on civilians.

The spike in violence on both sides of the border comes a year after the United Nations and African Union launched a new effort to get a political settlement for Darfur and a month after a joint A.U.-U.N. force took over peacekeeping duties in Darfur from a beleaguered A.U. force.

The U.N. envoy to Darfur, Jan Eliasson, and U.N. peacekeeping chief Jean-Marie Guehenno, told the Security Council on Friday that fighting in western Darfur and a dramatic deterioration in security throughout the Sudanese province have hurt prospects for a political settlement.