By Andrew Hammond and Sue Pleming

RIYADH/WASHINGTON, March 29 (Reuters) - Sudan has agreed to allow U.N. troops to join an African Union force in its troubled Darfur region, Saudi Arabia said on Thursday, but the United States voiced doubts as it readied tough new sanctions.

President Omar Hassan al-Bashir had long resisted the deployment of U.N. peacekeepers to the vast western province of Darfur, where Washington says a genocide has taken place through government support for nomadic militia groups.

"Sudan has now agreed for the U.N. to provide logistical support to help African forces," Saudi Foreign Minister Saud al-Faisal told a news conference at a summit of Arab nations.

"This is a breakthrough that never happened before and we hope it leads immediately to a solution to the humanitarian tragedy in Darfur as soon as possible."

A senior Bush administration official said Washington would wait to see whether Khartoum had indeed reversed course.

"We are very skeptical that Bashir has agreed to any such thing. We must see the fine print," the official told Reuters.

Before the Saudi announcement, U.S. officials from the State, Defense, Treasury and other departments had told Reuters that Washington would "tighten the screws" on Sudan with fresh measures, likely within days.

That would include a further limit on dollar transactions, said the officials, who spoke on condition of anonymity.

Beyond slapping travel and banking restrictions on at least three more individuals, including a rebel leader, Washington wants to put more pressure on fragmented rebel groups.

"You have to squeeze them all," said a defense official.

The United States also aims to pressure Bashir militarily by helping rebuild the forces of the Sudan People's Liberation Army which was at war with the north until a 2005 peace deal.

The defense official said military options like a no-fly zone over Darfur -- which Britain wants -- or a forced intervention had been ruled out for now but the Pentagon had done "back of the envelope" estimates on what might be needed.

NEW SANCTIONS 'NOT ENOUGH'

Experts say at least 200,000 people have been killed and 2.5 million displaced in Darfur since 2003, when rebel groups took up arms against the government, accusing it of neglect.

Khartoum says 9,000 people have died and denies the allegations of genocide.

The Saudi announcement came after Bashir met U.N. Secretary General Ban Ki-moon, Arab League chief Amr Moussa, Saudi King Abdullah and Kenyan President Mwai Kibaki, who heads an East African body, the Inter-Governmental Authority on Development.

Bashir had told Arab leaders at the summit's opening on Wednesday that the United Nations could have a role in providing logistical support for African troops.

A U.N. plan foresaw a small force of U.N. military and civilian forces moving into Darfur, followed in the second phase by about 2,500 more U.N. troops and then a further 10,000 soldiers to form a hybrid force.

Sudan, which has been accused of hindering aid to Darfur, signed an agreement with the United Nations this week to boost humanitarian work in the region.

Sudan said on Thursday the new U.S. measures would serve only to threaten that agreement and fuel violence in Darfur.

Some said the fresh sanctions were too little, too late.

"This is the right idea but it is simply not enough and not multilateral enough to make an impact, a dent, in the calculations of the Sudanese regime," said John Prendergast of the International Crisis Group.

The United States is working closely with Britain, which takes over the presidency of the U.N. Security Council next month, and is planning a new resolution on Darfur.

About 130 firms with ties to Sudan's government, including two oil companies, are already on a U.S. sanctions list barring them from doing business with the United States or using U.S. financial institutions to do dollar transactions.

Other companies would be added to the list, current sanctions would be tightened and loopholes closed.

"The goal is to be more pro-active and have tighter enforcement," said a Treasury Department official.

For the full story on the U.S. sanctions [ID:nN29225495] (Additional reporting by Kristin Roberts in Washington)

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