The prosecutor of the International Criminal Court says he has documented evidence of thousands of killings of civilians in Sudan's Darfur region.
The ICC prosecutor criticised Sudan authorities for their investigations
Luis Moreno Ocampo, investigating alleged crimes against humanity, says the killings include large-scale massacres and hundreds of rapes.
In a report to the UN, he also criticised Sudan's own investigations.
The report is seen as significant as the court may only prosecute if Sudan has not provided justice for victims.
Investigators from the ICC have not been able to travel to Darfur but they have managed to collect information about thousands of alleged murders in the western region of Sudan.
Unveiling his report, Mr Ocampo told the BBC his team had faced "serious obstacles".
He added: "We are now entering a new phase where unconditional co-operation will be essential to complete the investigation and identify those most responsible for crimes committed in Darfur."
A "Darfur crimes database", collated by investigators from the court, lists thousands of alleged murders, including massacres of hundreds of people at a time.
Eyewitnesses recounted the attackers saying things like: "We will drive you out of this land".
Mr Ocampo's report to the UN Security Council says some two million people have been displaced by violence in Darfur.
The first few months of 2006 saw increases in people forced from their homes in several areas of Darfur marked by increased violence, according to the document.
The Khartoum authorities insist they are continuing to set up courts and that the Sudanese judiciary will investigate allegations of abuse, but the ICC prosecutor says they do not appear to have done that.
But Mr Ocampo adds that the government has agreed to interviews with officials, to begin in August this year.
Some 200,000 people are thought to have died in Darfur, a vast region in the west of Sudan, in a three-year conflict.
Most have died in attacks by pro-government militias against civilians.
Rebel forces took up arms in February 2003, accusing the government of discriminating against Darfur's black Africans in favour of Arabs.
A partial peace deal was agreed in May, but not all sides signed the agreement and fears have grown over worsening conditions in camps home to displaced people.
The International Criminal Court was set up by the UN in 2002 to prosecute individuals for war crimes, genocide and crimes against humanity.