UNITED NATIONS — Despite a Sudanese promise yesterday to cooperate with one phase of a U.N. plan for Darfur, British and American officials are refusing to scale back their pressure on the government in Khartoum.

The Sudanese ambassador to the United Nations, Abdalmahmood Abdalhaleem, told Secretary-General Ban in a letter yesterday that his country agreed to the U.N. plan. Sudan had previously opposed the proposal, which calls for the deployment of 3,000 troops and six helicopter gunships in Darfur to support the current force of 8,000 African Union troops, as well as preparation for the next phase, in which a much larger force would be sent to the region.

But after a Security Council meeting here, Foreign Secretary Margaret Beckett of Britain said yesterday’s “little progress” on Darfur was the result of “diplomatic encouragement, but also the threat of sanctions.” She added that now “everyone is weighing up” whether that threat should be removed.

Western diplomats and U.N. officials said Sudan has made similar promises in the past and later broken them. The test for the country now will be “the whole implementation” of the U.N. plan, Undersecretary-General for Peacekeeping Jean-Marie Guéhenno told The New York Sun, adding that he still has concerns about “bureaucratic spokes they can put in it,” despite yesterday’s letter.

Mr. Abdalhaleem told reporters that the implementation of the agreement depends on U.N. funding for this phase of the plan, and on “enhancing the peace process” among his government, the militias it backs — known as the janjaweed — and the Darfur rebel groups.

The Sudanese ambassador stopped short of saying his government would accept the next phase of the plan, in which at least 20,000 African troops, backed by European and Western commanders and logistical support staff, would be deployed in an attempt to end the violence in Darfur, which America has called a “genocide.”

Deputy Secretary of State John Negroponte was in Sudan yesterday, and several diplomats said easing the threat of U.N. Security Council sanctions and other decisions would depend largely on his assessment of Sudan’s readiness to cooperate with the U.N. plan.

In London, Secretary of International Cooperation Hilary Benn said Britain and its allies are still preparing for an “extended arms embargo” against Sudan and “sanctions on individuals.”

Khartoum has yet to accept the full deployment of a “hybrid force” of African and U.N. troops, agreeing only to a “transitional” phase, an American ambassador to the United Nations, Alejandro Wolff, said. “We have seen a letter from the Sudanese ambassador,” he said, but “learned a long time ago not to take these letters at face value.”

The first reports of a possible Sudanese policy reversal appeared yesterday by the Saudi Press Agency, which reported that President al-Bashir of Sudan had informed King Abdullah of Saudi Arabia that he had signed an agreement “determining the duties of the African Union and United Nations forces in Darfur.”

At a recent Arab League summit, Mr. Ban attempted to enlist the king and other Arab leaders, who in the past have declined to put pressure on Sudan, to convince Khartoum to accept the U.N. plan. Yesterday, Mr. Ban thanked the king.