Khartoum, Sudan ? Sudan gave African Union troops a one-week ultimatum Monday to accept a deal blocking a proposed U.N. peacekeeping force in Darfur or else leave the war-torn region, a step that would likely worsen the world’s worst humanitarian disaster.

The deadline escalates the Khartoum regime’s standoff with the United Nations over Darfur to a crisis point, pitting Sudanese determination to resist possible war crimes investigations against a U.N. push to take on a new, tough peacekeeping mission.

Many observers believe Sudan has dug in against U.N. deployment in the vast western region because it fears the force will hunt down officials and government allies suspected of war crimes for atrocities committed by nomadic Arab tribes on Darfur’s ethnic African communities.

The removal of the AU peacekeepers would increase chances for a surge of bloodshed in the arid region, where fighting stemming from long-standing disputes over land and water has killed some 200,000 people since 2003 and chased an estimated 2.5 million people from their homes.

After ethnic Africans revolted against the Arab-led Khartoum regime, the government allegedly unleashed Arab militiamen known as janjaweed who have been blamed for widespread atrocities. The United States has described the rapes, killings and other attacks as genocide.

Aid workers say the conflict has worsened in recent months, and U.N. officials warn that hundreds of thousands of people could die if aid operations collapse. Twelve aid workers have been killed in Darfur this year, most in the last two months.

The AU force, which is underfunded and undermanned, has struggled to keep stability amid the recent violence, so the United Nations wants to deploy a beefed up force of 20,000 soldiers with a stronger mandate to stop the fighting.

The African peacekeepers’ mandate runs out Sept. 30, and last week the Security Council passed a resolution that would put the AU force under U.N. control, pending Sudan’s consent. The government in Khartoum promptly rejected the resolution.

On Monday, Sudan went a step further. Foreign Minister Ali Ahmed Kerti said the AU force can remain in Darfur only if it accepts Arab League and Sudanese funding. He gave the African Union a week to agree or get its troops out, a government statement said.