At a landmark review conference of the ICC in Kampala, delegates are seeking to agree a definition of state aggression and how ICC investigations into the crime, one of four grave crimes the court has jurisdiction over, could be triggered.

The issue has divided delegates and NGOs over fears that giving the court powers to prosecute state aggression -- defined broadly as using force that manifestly breaches the UN charter -- could open it up to criticism of political bias and may again prove too divisive for full agreement to be reached in Kampala.

United States ambassador-at-large for war crimes Stephen Rapp warned late Tuesday about legal uncertainties over state aggression investigations and said that that pushing forward on the issue despite a lack of "genuine consensus" could undermine the ICC.

"What impact might the proposed definition, if adopted, have on the use of force that is undertaken to end the very crimes the ICC is now charged with prosecuting?" he said.

The United States withdrew its support for the ICC under then president George W. Bush in 2002, worried that its troops could face politically motivated prosecutions over unpopular wars, but has more recently started to re-engage with the court.