The United Nations launched the largest peacekeeping mission in the world last night in an attempt to halt more than four years of massacres that have cost almost a quarter of a million lives in the Darfur region of Sudan.

After months of tortuous negotiation, the UN Security Council adopted unanimously a landmark resolution, co-sponsored by Britain, to create a 26,000-strong “hybrid” UN-African Union peacekeeping force.

The force, to be composed primarily of African troops, will be empowered to “take the necessary action” to prevent attacks and protect civilians in the vast desert region that has been racked by violence since 2003.

Gordon Brown hailed the move during a stop at UN headquarters on his way home from his summit with President Bush. He threatened to seek further sanctions if Sudan failed to co-operate. “This is the world coming together to say that we have a plan now, that we expect the authorities in Sudan to act. We will not tolerate further inaction, and the violence has got to stop now,” Mr Brown said.

The resolution authorises a force of 19,555 troops plus 6,432 civilian police to take over from the overstretched 7,000-strong African Union peacekeeping operation in Darfur by the end of the year.

The hybrid force will try to quell the violence, which began with camelriding Arab Janjawid militia slaughtering black villagers but has now fragmented to involve a patchwork of rebel groups and feuding tribes and has spilt over the borders into Chad and the Central African Republic.

“If Sudan does not comply with this resolution, the United States will move for the swift adoption of unilateral and multilateral measures,” Zalmay Khalilzad, the US Ambassador to the UN, said. “Now Sudan faces a choice. Sudan can choose the path of co-operation or defiance.”

Diplomats had been working to secure Sudan’s agreement to an expanded peacekeeping force. A British-sponsored resolution in 2006, authorising a larger UN force of 17,300 troops plus 3,300 police, came to nothing when Sudan refused to consent.

A summit in Addis Ababa last November devised a new three-tiered proposal, starting with the UN providing first “light support” and then “heavy support” to the African Union operation before establishing the UN-AU force. The UN is still struggling to deploy the “heavy support” package of 2,250 troops, 750 police and six attack helicopters.

Abdalmahmood Abdalhaleem Mohamad, the Sudanese Ambassador to the UN, said that his Government — which includes a minister for humanitarian affairs who has been indicted by the International Criminal Court for alleged war crimes in Darfur — would implement the resolution establishing the “hybrid” force. He called for renewed efforts to reach a political settlement, after the rejection of the May 2006 Darfur Peace Agreement by two of the three rebel groups that negotiated it. “Peacekeeping should be assisted fully by rejuvenation of the political process,” he told The Times. “There should be a peace to keep.”

Jan Eliasson, the UN mediator, from Sweden, and Salim Salim, of Tanzania, his AU counterpart, are hosting a meeting in the Tanzanian city of Arusha on Friday to try to persuade non-signatory rebels to renew negotiations with the Sudanese Government.

“The plan for Darfur is to achieve a ceasefire, including an end to aerial bombings of civilians; drive forward peace talks starting in Arusha, Tanzania, this weekend on August 3; and as peace is established to offer to and begin to invest in recovery and reconstruction,” Mr Brown said at the UN. “But we must be clear: if any party blocks progress and the killings continue, I and others will redouble our efforts to impose further sanctions.”

The UN maintains an arms embargo on Darfur and individual travel bans and asset freezes against four individuals — a former air force commander, a Janjawid militia chief and two Darfurian rebel commanders. Britain and the US have threatened to seek an expanded arms embargo covering all of Sudan and to add the names of more individuals and companies to the UN blacklist if Khartoum fails to co-operate.

The new force will overtake the 17,000-strong operation in the Democratic Republic of Congo to become the largest UN peacekeeping mission, costing an estimated $2.6 billion (£1.3 billion) a year. It will run in parallel with the existing UN peacekeeping mission in southern Sudan, which comprises about 16,000 personnel and costs $1 billion a year.

At Sudan’s insistence the new force will have a “predominantly African character”. China, an ally of Sudan, signalled yesterday that it may be willing to provide peacekeeping troops. China has already pledged 275 soldiers for the UN’s “heavy support” package in Darfur and could provide engineers to help to establish the new force. The Chinese Ambassador to the UN said that Beijing would send more troops if it were approached.

China buys two thirds of Sudan’s oil and is facing calls for a boycott of the 2008 Beijing Olympics for not applying enough pressure on the Sudanese Government over Darfur.

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