UNITED NATIONS — The Security Council yesterday unanimously approved the creation of a 26,000-strong force to be sent to Darfur to help safeguard civilians and to begin to provide aid in the aftermath of the atrocities committed in the war-ravaged region. It was a move relished by Prime Minister Brown of Britain, lauded by Secretary-General Ban, and cautiously embraced by the Sudanese government.

The largest U.N. force ever to be implemented, it is designed as an experiment in cooperation between the world body and the African Union. The so-called hybrid force is to “assume responsibility” in Darfur by the end of the year, according to the new resolution. By then, the mostly African force of 20,000 troops and 6,000 police guards — with a command structure controlled by the United Nations — would have to be in place, financed, and fully armed. Early estimates put the cost of the new force’s first year of operation at more than $2.6 billion — “but that is assuming that all the troops are deployed,” a U.N. official said, speaking on condition of anonymity.

“The ball is now in your court,” Sudan’s ambassador to the United Nations, Abdalmahmood Abdalhaleem Mohamed, said, indicating a sense in Khartoum that the West would be slow to finance the new force — despite its talk of stopping the humanitarian catastrophe in Darfur — and that African troops would be slow to arrive.

“We must now move forward in all haste,” Mr. Ban, who played an instrumental role in convincing Khartoum to accept the hybrid force, said. “Additional capable troops must be committed. Support systems must be put in place. Command structures must be established. National governments know from their own experience that this takes time, but time is not our side.”

Westerners, however, expressed doubt about Khartoum’s willingness to abide by the agreements it has signed. Mr. Brown, who visited the United Nations yesterday for a speech on international development, said that “if any party blocks progress and the killings continue, I and others will redouble our efforts to impose further sanctions.” Not waiting for the United Nations to lead, the House of Representatives yesterday passed the “Darfur Accountability and Divestment Act of 2007” in a 418–1 vote. The legislation requires the U.S. Treasury Department to publish “a list of companies or entities whose business dealings directly benefit the regime in Khartoum,” and it “enables State and local governments to divest from those companies, and provides protection to fund managers from lawsuits brought by investors who disagree with any decision to divest.” And although America’s ambassador to the United Nations, Zalmay Khalilzad, urged reporters to “not read a lot into this,” America refrained from sponsoring the resolution, which was initiated by France and Britain. An American diplomat told The New York Sun that Washington urged the sponsors to include a faster transition of authority to the new hybrid force from the African Union force currently on the ground. America also wanted the part of the resolution that referred to possible sanctions to be clearer, in case of noncompliance.

Mr. Khalilzad said he hoped President Bashir’s acceptance of the force “marks a new chapter in his cooperation with the international community,” but Mr. Khalilzad added, “If Sudan does not comply with this resolution, the United States will move for the swift adoption of unilateral and multilateral measures.”

European sponsors compromised on the language in order to gain the cooperation of the Sudanese government. “The whole of Sunday, I was with Ambassador Emyr, cleaning up the text,” Sudan’s ambassador to the United Nations, Mr. Mohamed, told reporters yesterday, referring to his weekend meeting with Britain’s U.N. envoy, Emyr Jones Parry.

At least one murky line referring to protection of civilians may prove difficult for the troops on the ground to interpret. This line instructs the hybrid force to do all it can to protect civilians “without prejudice to the responsibility of the Government of Sudan.”

When asked who would decide whether protection is needed — the hybrid force commanders or the Sudanese army — Mr. Mohamed said yesterday that he did not want to get into conflict with the United Nations before the force was even deployed yet.