LONDON – Nigeria warned Sudan yesterday that it risked international ostracism after cease-fire monitors confirmed reports of a recent attack by Khartoum’s forces against civilians in Darfur.

The rare public admonishment of a fellow African country by Nigeria came on the day that the United Nations deadline for Khartoum to stop the ethnic cleansing of refugees in Darfur was due to expire.

But the Sudanese government, which hopes for a “reasonable decision” by the Security Council next week, rejected claims that it was responsible for the violence. It accused Darfur rebels of kidnapping eight U.N. and Red Crescent workers.

An estimated 50,000 people have died and more than a million civilians have been driven from their homes in Darfur in the past 18 months.

The long-running conflict between settled villagers and nomadic tribes turned into one of the world’s worst humanitarian crises after two rebel groups took up arms, while Khartoum backed the Janjaweed militia in an ethnic cleansing campaign against villagers.

Roman Catholic bishops in Sudan said yesterday the atrocities constituted genocide.

With the Security Council warning Sudan to restore security or face sanctions, the Nigerian president, Olusegun Obasanjo, is searching for an “African solution.”

He is hosting peace talks between the Khartoum government and the rebels, and Sunday a company of 150 Nigerian soldiers left for Darfur. The troops will join a similar number of Rwandan forces to act as ceasefire monitors for the African Union.

Mr. Obasanjo’s office said he had written to the president of Sudan, Omar el-Bashir, to “express his disappointment at the reported attacks which he said had been confirmed by the chairman of the cease-fire monitoring commission.”

Mr. Obasanjo asked Sudan to “ensure that all attacks by government forces and the Janjaweed cease forthwith.”