The United States is concerned about how aggression will be defined as an international crime.

"I would be remiss not to share with you my country's concerns about an issue ... to which we attach particular importance: the definition of the crime of aggression," US war crimes ambassador at large  Stephen Rapp told a gathering in The Hague of the International Criminal Court's Assembly of State Parties (ASP).

The court's founding Rome Statute, of which the US is not a signatory, determines that the ICC can try aggression, though no legal definition has been agreed upon.

The issue is to be discussed at an ICC review conference in Kampala, Uganda, next May.

Rapp, who made no allusion to the United States ratifying the statute in the future, said Washington was concerned about the way a draft definition of aggression had been framed.

"Our view has been and remains that, should the Rome Statute be amended to include a defined crime of aggression, jurisdiction should follow a Security Council determination that aggression has occurred."

He declined to elaborate when approached afterwards.

The United States is taking part in a meeting of the ASP, as an observer, for the first time since the world's only permanent court for genocide, war crimes and crimes against humanity started operating in 2002. 

The ASP, the court's management and oversight body, is made up of representatives of all 110 member parties.

Former president George W. Bush had fiercely opposed the ICC, fearing it could target Americans out of political bias considering US dominance around the world.

Rapp said "the commitment of the Obama administration to the rule of law and the principle of accountability is firm."

On Wednesday, ASP president Christian Wenaweser welcomed the US presence as "a very important gesture."