UNITED NATIONS - Sudan agreed Monday to let 3,000 UN peacekeepers deploy in Darfur with attack helicopters, opening the door to the first significant UN force to help beleaguered African Union soldiers who have been unable to halt the region's four-year war.

Secretary-General of United Nations Ban Ki-Moon, right, greets Oumar Konare, left, chairman of the African Union and former president of Mali at the beginning of an informal meeting regarding Sudan at UN headquarters on Monday, April 16, 2007. [AP]

After five months of stalling, the government in Khartoum called for a speedy deployment and hinted it could approve an even larger UN force that has been demanded by the UN Security Council, the United States and others.

But experts were cautious about chances for creating that 20,000-strong force, noting Sudan's leaders have reversed course previously after announcing vague agreements for action in Darfur.

UN Secretary-General Ban Ki-moon called Monday's agreement "a very positive sign" and said the UN and the African Union would "move quickly" to put together the 3,000 peacekeepers as well as press for a deal allowing the larger force.

The United Nations has no standing army and Ban is urging UN member states to contribute troop and police quickly for Darfur, but it will likely take months before the UN contingent is on the ground.

The Security Council reacted swiftly, welcoming Sudan's decision and calling on Khartoum to facilitate "the immediate deployment" of the force. It also called for "an immediate cease-fire, a reinvigorated political process, (and) an improvement in the humanitarian situation."

British Ambassador Emyr Jones Parry, the current council president, said he sent a letter to Ban late Monday on behalf of the council asking the secretary-general to seek General Assembly approval for financing the UN force.

Sudanese President Omar al-Bashir, whose Arab-dominated government has been accused of aiding Arab militias fighting ethnic blacks in Darfur, had long opposed a UN force to help the weakly armed 7,000-soldier African Union peacekeeping mission.

But he came under increasing pressure from the US, the European Union, some Arab and African countries and China.

Though al-Bashir's government has a history of finding loopholes in agreements with the UN and others, the deal reached Monday appeared to be a meaningful step forward in bringing a more effective peacekeeping force to Darfur.

Nonetheless, US diplomats and UN officials said they would remain cautious until UN peacekeepers were on the ground in Sudan's vast western province, where more than 200,000 people have been killed and 2.5 million chased from their homes since the conflict began in 2003.

"We'll see whether they've agreed when they actually start to deploy," acting US Ambassador Alejandro Wolff told reporters.

The State Department said the announcement omitted several key provisions for the UN force's effective operation, including leaving its command and control unspecified and limiting the participation of non-African troops.

"While it is a partial step forward, it certainly does not meet all the requirements," spokesman Sean McCormack told reporters. "There are still elements and other caveats that remain in place."

McCormack could not say if Deputy Secretary of State John Negroponte, who was in Sudan over the weekend to discuss Darfur, had been told of the Sudanese decision to accept a UN force before it was announced.

Speaking at the end of his three-day visit, Negroponte told reporters in Khartoum that Sudan faced "continued and possibly even intensified international isolation" if it did not move quickly to implement UN plans to strengthen the AU peacekeeping mission, improve aid agencies' access to Darfur and beef up the region's transitional authority.

Darfur's war began when groups based in black farming communities rebelled, accusing Khartoum of discriminating in favor of nomadic Arab tribes in disputes over land and water. The AU force arrived in 2004 but is too weak to impose calm, even having seven of its own men killed this month.

After long wrangling, Sudan accepted the UN plan to send 3,000 UN soldiers, police officers and other personnel along with logistical and aviation equipment, including six attack helicopters that the Sudanese government previously rejected.

"The government has agreed upon the entire `heavy assistance package' by the United Nations to the African force in Darfur," Sudanese Foreign Minister Lam Akol told journalists in Khartoum.

Sudan's agreement came after China applied pressure on its trading partner. During a visit in February, Chinese President Hu Jintao urged Sudan to give the UN a bigger role in Darfur, and China's assistant foreign minister called last week for Sudan to accept UN peacekeepers.

China itself was being pressured by critics who accused the communist regime, which is one of five veto-holding nations on the Security Council, of protecting Sudan from strong UN action. Western activists used China's hosting of the 2008 Olympics to embarrass its government by branding the games the "Genocide Olympics."

Actress Mia Farrow, a goodwill ambassador for UNICEF, the UN children's agency, who was one of those urging Olympics sponsors to pressure China over Darfur, said the link to the games had an effect.

"We are amazed by Beijing's reaction," Farrow told The Associated Press by phone from the US "It shows that one thing is more important to the Chinese than their access to Sudan's oil, and that's the success of their Olympic Games."

Sudan's UN ambassador, Abdalmahmood Abdalhaleem, informed the secretary-general of his government's Darfur decision and said in a letter that Khartoum hoped "implementation of the heavy support package would proceed expeditiously."

The letter arrived as Ban began two days of meetings with the African Union's chief executive, Alpha Oumar Konare, and two diplomats who are trying to promote a political settlement in Darfur - UN envoy Jan Eliasson and AU official Salim Ahmed Salim.

Konare told reporters late Monday he was "delighted" with Sudan's announcement, but stressed that the African Union needs international funding to keep its own troops in Darfur.

"Let's be honest, without any sustainable financing, all this operation might not be as successful as expected," he said.

Under a November agreement, the UN already has deployed a small force of UN police advisers, civilian staff and additional resources and technical support in Darfur.

Now that the second force has been accepted, UN and AU officials will focus on negotiating a third stage of deployment - creating a joint AU-UN force with 17,000 soldiers and 3,000 police officers.

Although that was part of the November agreement, Al-Bashir has backed away from the third stage, saying he would only allow a larger AU force, with technical and logistical support from the United Nations.

Abdalhaleem told reporters late Monday the hybrid force "should be an African forces, African command, with UN backstopping in techniques of control and command."

Getting Sudan to accept the 20,000-strong force would be a major breakthrough, but experts warned it won't come easily, or quickly.

"This sounds like a very decisive step forward," Tom Cargill, a Sudan specialist at Chatham House, a think tank in London. "But I've become so, so skeptical of any announcement made by the Sudanese government ... They've agreed so many times to things, and then backtracked again and again and again."