The two men were hanged at dawn on January 15.
European Commission President Jose Manual Barroso also condemned the hangings, during which the head of Hussein's half-brother, Ibrahim Barzan al-Tikriti, was severed from his body.
Amnesty International called the execution brutal and cruel.
The U.S. ambassador to Iraq, Zalmay Khalilzad, said the hangings were "an Iraqi decision, an Iraqi execution."
Meanwhile, Secretary-General Ban is due to meet U.S. President George W. Bush today at the White House.
(compiled from reports)