Facing a possible veto by the United States, the UN Security Council was struggling last night to adopt a resolution that would authorise the fledgling International Criminal Court in The Hague to try war crimes suspects from the Darfur conflict in Sudan.

Diplomats said that the Council was likely to put the French-drafted resolution to a vote before nightfall and thus bring six weeks of wrangling between members to an end. However, desperate last efforts were being made to overcome strong opposition to the text from Washington.

Britain and a majority of the Council's members have become frustrated with the American position. A veto by the US envoy would kill a resolution that is seen by many as vital to efforts to contain violence in Darfur which, according to a committee of British MPs, has killed up to 400,000 people.

The passage of the resolution would also mark a huge milestone for the court, established in 2002 precisely to pursue the kinds of atrocities that have been identified in Darfur. It has not yet tried a case.

The United States is not a signatory to the treaty that created the court, however, and the administration of President George Bush fears that acceptance of the United Nations text would signal tacit support for it. That would almost certainly cause a backlash against the White House by conservative Republicans.

Frantic negotiations over recent days, involving both Jack Straw, the Foreign Secretary, and his US counterpart, Condoleezza Rice, have been aimed at finding language to ensure that all Americans in Sudan, whether military or civilian, would be immune from prosecution by the court. But Washington is even unwilling to accept language that would expect the US at least to co-operate with the court's efforts.

But while Britain, France and many other council members were eager to avoid a veto by the US, they similarly did not want to see a resolution that might appear to weaken the standing of the court.

"All along this has been a game of chicken," said Richard Dicker, an expert of the court with Human Rights Watch. Underscoring the importance of the resolution, he added: "The Council has to send the signal to Sudan that there will be serious efforts to prosecute those guilty of war crimes there."

Long accused of inaction over Sudan, the Security Council in recent days has adopted two other resolutions both to strengthen sanctions against Sudanese officials and the government as well as to deploy 10,000 peacekeepers to monitor a ceasefire between government forces and rebels in southern Sudan. However, it is the resolution on war crimes that might have the greatest impact on ending the carnage.

Indeed, the embarrassment over the Council's failure to adopt the resolution has been mounting. It is a diplomatic stand-off that started in early February. But it did appear yesterday that the willingness of the majority supporting the resolution to entertain further American delay had run out.

There had been plans for a vote on the resolution on Wednesday but, at the very brink of being forced to cast a veto, the US asked for another 24 hours to investigate possible amendments that would at least allow its ambassador to abstain. There is no suggestion that the US will ever vote in favour.