The US and British governments are gathering evidence to determine whether genocide is being committed in the Darfur region of Sudan, where an estimated 30,000 people have been killed and more than a million have fled their homes.
The Foreign Office said yesterday that it would not shy away from uncomfortable conclusions, even though a declaration of genocide would invoke a legal obligation to intervene.
The UN security council is preparing to vote on a resolution warning Sudan to protect civilians or face sanctions in 30 days and in the meantime putting a weapons embargo on armed groups in Darfur.
Sudan has bridled at the increasing international pressure and the suggestion by General Sir Michael Jackson, the chief of general staff, that Britain would be able to send a force of 5,000 if necessary.
Yesterday its foreign minister, Mustafa Osman Ismail, said Sudan would retaliate if troops were sent.
The US secretary of state, Colin Powell, said it was "premature" to start talking of a military intervention, adding that the Sudanese government "still has the ability to take action and bring this violence under control".
In Britain a Foreign Office spokesman asked whether what was happening in Darfur amounted to genocide said: "There are certainly some elements. There is an ethnic element to the violence, but we do not at the moment have incontrovertible proof."
Ceasefire monitors have documented continuing instances of attacks on Darfur villages. Monitors from the African Union said that in an incident three weeks ago militiamen killed villagers by chaining them and burning them alive, Reuters reported. The Foreign Office is collating information from various sources in Darfur, and US officials have been interviewing Sudanese refugees who have crossed the border into Chad, to determine whether genocide is taking place.
Both governments are keen to keep up the diplomatic pressure on the Sudanese government in the hope that international action will not be needed. But they fear being accused of failing to act, as they were in Rwanda and in Srebrenica. Genocide is defined in a 1948 UN convention as acts "committed with intent to destroy, in whole or in part, a national, ethnic, racial or religious group".
These acts include: killing members of the group; causing serious bodily or mental harm to members of the group; and deliberately inflicting on the group conditions of life calculated to bring about its physical destruction in whole or in part.
The US House of Representatives passed an unopposed motion last week declaring genocide in Darfur.
The Senate unanimously agreed in a voice vote.
The crisis began last year when the Sudanese government armed militia groups, known as the Janjaweed, to help suppress a rebellion.
A Foreign Office source said three tribes were being targeted the Fur, the Masalit and the Zaghawa but all three were also contributing members to the the two rebel movements, the Sudan Liberation Army and the Justice and Equality Movement. The source said: "At what point does civil war become genocide? We can see there is violence against these tribes by the army and the militia. It is not at the stage yet of a concerted attempt to wipe them out."