After months of delay, Sudan said it would let 3,000 UN peacekeepers into Darfur to reinforce an African Union force struggling to keep the peace, the most substantial agreement to date.
But the new military personnel may take six months to recruit and deploy, UN officials said. And Sudan has not yet agreed to a larger force of more than 20,000.
Sudan's acceptance on Monday of an interim plan is also expected to put in abeyance US-British plans to introduce sanctions against Khartoum so Sudan has a chance to carry out the pact, diplomats said.
Still, the United States voiced skepticism, noting that Khartoum had reneged on previous agreements on letting troops into Darfur, where at least 200,000 people have been killed and 2.5 million have been uprooted from their homes.
"We've been down this path before," said acting US Ambassador Alejandro Wolff. "So we will see if it happens when it happens." He said no decision had been made on sanctions.
UN Secretary-General Ban Ki-moon called Monday's agreement "a very positive sign" and said the United Nations and the African Union would move quickly to recruit the peacekeepers, who would man control centers but not join infantry units.
Jean-Marie Guehenno, in charge of UN peacekeeping, told reporters he would meet on Thursday with potential troop contributors, many of whom are hesitant about the future of the force, which would serve under African Union command.
The deal with Sudan has three stages: a light support package with UN police advisers, civilian staff and equipment, which has nearly been deployed.
The second phase is the so-called heavy support package, which Sudan approved on Monday, including six attack helicopters that Khartoum had opposed until the last minute.
And the third stage is a peacekeeping force of more than 20,000 troops and police which Sudan has not accepted.
For Guehenno, Monday's agreement is a prelude to the larger force. "This is not the robust force that Darfur needs. It's a support package to lay the groundwork for a future robust force," he said.
Sudan has made clear it expects the third phase, a so-called "hybrid" force to include only African Union infantry troops.
But the United Nations and the African Union said other nations would be called on if there were not enough Africans.
Visiting African Union Commission Chairman Alpha Oumar Konare, after briefing UN Security Council members, said there were two African battalions ready to go to Darfur. But he said they needed financing for their upkeep and for weapons.
"Let us be honest, without any sustainable financing this operation might not be successful, Konare said.
Monies will have to come from voluntary donations until the UN force arrives, at which time all UN members will be assessed for the financing. But first Sudan needs to allot land for camps and provide water needs.
In Khartoum, John Negroponte, the US deputy secretary of state, told reporters, "We must move quickly to a larger hybrid United Nations-African Union peacekeeping force with a single unified chain of command that conforms to UN standards and practices."
Negroponte urged Khartoum to disarm militias accused of some of the worst assaults against civilians in Darfur. The militias, he said, "could not exist without the Sudanese government's active support."