Rebel leaders from Sudan's Darfur region are being investigated for war crimes, says International Criminal Court's prosecutor Luis Moreno-Ocampo.
Mr Ocampo last week asked for Sudanese President Omar al-Bashir to be indicted on war crimes charges, sparking angry protests in the capital, Khartoum.
Now he wants rebels to help investigate two of their own commanders accused of attacking African Union peacekeepers.
"Any attack against peacekeepers... is a war crime," he told the BBC.
Rebels are accused of killing 10 AU peacekeepers last year in an assault on their base in the town of Haskanita, in southern Darfur.
Mr Ocampo told the BBC he was asking other militant groups to help him gather evidence against two rebel commanders suspected of leading the attack.
"The rebel groups have an opportunity to... provide evidence, to arrest the commanders and to stop crimes," he said, adding that the rebels had to show respect for the law.
The ICC prosecutor rejected suggestions that he had jeopardised peace efforts in Darfur by asking for an arrest warrant for Mr Bashir.
He says the UN Security Council decided justice had to be done in Darfur to make peace.
"A few weeks ago, in June 2008, the Security Council confirmed the need to do justice", he said, "so I am doing what the Security Council requested me to do."
Some UN officials fear reprisals against UN peacekeepers and aid workers in Darfur, if an arrest warrant is issued against Mr Bashir - some non-essential staff have been pulled out of the region.
There are some 9,000 members of a joint UN-African Union peacekeeping force in Darfur, of a planned 26,000 contingent.
The five-year-old conflict has left some 300,000 people dead, while more than two million have fled their homes, the UN says.
Sudan's government says the scale of the violence and suffering has been exaggerated by the west for political reasons.
It denies charges that it organised the Arab Janjaweed militias, accused of widespread atrocities against Darfur's black African population.