UNITED NATIONS - UN Secretary-General Kofi Annan said on Wednesday he was encouraged by Sudan's acceptance of a hybrid African Union-UN force for the troubled Darfur region but the test would lie in its implementation.

Annan briefed the UN Security Council on a letter from Sudan President Omar Hassan al-Bashir in which Khartoum endorsed a three-step UN proposal to strengthen the understaffed African Union force of 7,000.

The third step is a hybrid AU-UN force but Annan said the number of troops would be worked out by a joint African- United Nations team. The council has authorized up to 22,500 troops and police.

"The president has accepted the three-phased approach as a package, and we will have to move ahead and implement it, push it, and -- even those who have some doubts -- to test it and push it very quickly," said Annan, who ends 10 years in office on Sunday.

But he warned against delays and disappointments in deploying a UN force, which has been opposed by Sudan. UN plans have included African and Asian troops on the ground, with Europeans helping in command centers and in the air.

"Obviously, when there have been so many disappointments, it is only natural that there will be some doubts and hesitations," Annan said.

"And this is a challenge for the Sudanese government to prove to the international community that it means business, that it stands by the letter that it has written to me, he said. "So we are going to press ahead and I hope this time there will not be disappointments."

Non-Arab rebels took up arms in Darfur in early 2003, accusing central government of marginalizing the remote western region on the border with Chad. To quell the revolt, Khartoum armed militia, who embarked on a campaign of rape, murder and pillage. Some 2.5 million people have been uprooted and now are terrorized by rebel factions also.

Annan said the question of the color helmets -- green for the African Union, blue for the United Nations -- also had been resolved. UN troops can wear blue with an armband from the African Union, which will name the commander.

US SKEPTICAL

Several diplomats were skeptical. The acting US ambassador, Alejandro Wolff told reporters, "The proof of this will be action on the ground and whether or not we will actually see a force that can take its place in Darfur to address the humanitarian crisis there and help deal with the fighting."

But Wolff said, "If this letter and the commitments laid out in the letter by the president result in the deployment of the long-awaited hybrid force of the United Nations and the African Union -- then indeed it will be a welcome development."

After launching an intense media campaign against UN troops being deployed to Darfur, the Khartoum government is now believed to be trying to save face domestically by not announcing at home it would accept a joint force.

The United Nations has some 10,000 peacekeepers in southern Sudan to monitor a peace agreement there and help train police, human rights workers and provide other services.

But the peacekeepers have not been permitted to monitor the oil-rich Abyei region. Sudan customs also has delayed the release many items, including food rations" and held some major communications equipment for months, the United Nations reported earlier this year.