The UN Security Council unanimously approved a 26,000-strong peacekeeping force for Darfur on Tuesday to try to help end four years of fighting that has killed more than 200,000 people in the vast Sudanese region. The force - the first joint peacekeeping mission by the African Union and the United Nations - will replace the beleaguered 7,000-strong AU force now in Darfur no later than Dec. 31. UN Secretary-General Ban Ki-moon said the resolution will send "a clear and powerful signal" of the UN's commitment to help to the people of Darfur and the surrounding region "and close this tragic chapter in Sudan's history." But Ban, who has made Darfur a top priority since taking over as UN chief on Jan. 1, stressed that "it is only through a political process that we can achieve a sustainable solution to the conflict." The secretary-general said it is crucial that a meeting of the parties to the conflict in Arusha, Tanzania, later this week, "yield positive results so as to pave the way for negotiations and, ultimately, a peace agreement." Sudan's UN ambassador, Abdalmahmood Abdalhaleem Mohamad, told reporters the government would discuss the resolution, which "contained many positive elements, and also it went to considerable extent to satisfy our concerns." He had reacted harshly to earlier versions of the resolution, calling one circulated last week "ugly" and "awful." Britain and France, the key sponsors of the resolution, then stripped some tough language including the threat of sanctions which Sudan and China opposed. US Ambassador Zalmay Khalilzad warned that if Sudan doesn't comply with the resolution "the United States will move for the swift adoption of unilateral and multilateral measures." "Now Sudan faces a choice. Sudan can choose the path of cooperation or defiance," he said. "We look to its government to do the right thing and pursue the path of peace." Earlier Tuesday, Britain's new prime minister, Gordon Brown, told a U.N. audience that "if any party blocks progress and the killings continue, I and others will redouble our efforts to impose further sanctions." Britain's UN Ambassador Emyr Jones Parry said the AU-UN force will be the largest peacekeeping force in the world. The conflict in Darfur began in February 2003, when ethnic African tribes rebelled against what they consider decades of neglect and discrimination by the Arab-dominated government. Sudan's government is accused of retaliating by unleashing a militia of Arab nomads known as the janjaweed - a charge it denies. The poorly equipped and underfunded African Union force has been unable to stop the fighting, and neither has the Darfur Peace Agreement, signed a year ago by the government and one rebel group. Other rebel factions called the deal insufficient, and fighting has continued. The UN and Western governments have pressed Sudan since November to accept a three-stage U.N. plan for a joint force. After stalling for months, Sudanese President Omar al-Bashir agreed in April to a "heavy support package" to strengthen the AU force, including 3,000 U.N. troops, police and civilian personnel along with aircraft and other equipment. The resolution calls for its speedy deployment. The resolution authorizes the much larger 26,000-strong hybrid force, which will be called UNAMID and have "a predominantly African character," as Sudan demanded. The force will have up to 19,555 military personnel, including 360 military observers and liaison officers, a civilian component including up to 3,772 international police, and 19 special police units with up to 2,660 officers. Estimates vary widely on the combined size of the two main rebel groups operating in the region, the Sudan Liberation Army and Justice and Equality Movement. Jane's Information Group puts the number at 10,000 fighters, while the Washington, DC-based Center for Strategic and International Studies estimates around 1,200 to 2,000. The Janjaweed militia peaked at about 10,000 fighters with the Brussels-based International Crisis Group saying one faction is believed to have about 3,000 to 4,000 under arms. Jane's said in 2005 that the overall size of Sudan's national army was 94,300 troops. The resolution calls on UN member states to finalize their contributions to the hybrid force in 30 days, and the UN and the AU to agree on the final military composition during the same period. The final draft has one section under Chapter 7 of the UN Charter, which deals with threats to peace and security and can be militarily enforced. It authorizes UNAMID to take "the necessary action" to protect and ensure freedom of movement for its own personnel and humanitarian workers. It also authorizes the hybrid force to take action to "support early and effective implementation of the Darfur Peace Agreement, and prevent the disruption of its implementation and armed attacks, and thus to protect civilians, without prejudice to the responsibility of the government of Sudan." There is "nothing enjoyable in these type of things but we will deliver our commitments," Mohamed, the Sudanese envoy, said when asked about Sudan's reaction to the mandate for the new force.