Both Mr Kenyatta and Mr Ruto have repeatedly called for the cases against them to be dropped, saying the charges are politically motivated.
The ICC has refused and says it pursues justice impartially.
In May, the African Union accused the ICC of "hunting" Africans because of their race.
The ICC strongly denies this, saying it is fighting for the rights of the African victims of atrocities.
The ICC was set up in 2002 to deal with genocide, crimes against humanity, war crimes and the crime of aggression.
The court has been ratified by 122 countries, including 34 in Africa.