"There been a delay because one of the defendants is refusing to come up to the court. That defendant is now meeting with his attorneys," the court source said, according to an AFP report.
"If he doesn't come they come up they will make other arrangements," the official added, later naming the defendant as Saddam.
On Tuesday, Saddam threatened to boycott the trial after five witnesses gave evidence of suffering brutalities during a bloody crackdown in the village of Dujail in 1982.
Saddam's parting words to the court were: "Go to hell".
Under Iraqi law, individuals can be tried in absentia, but the source said presiding judge Rizkar Mohammed Amin would want to make sure the defendant is involved.
Early in the morning, 20 gunmen stormed into a hospital in the northern city of Kirkuk to free a detainee from an Islamist cell planning to assassinate one of the judges trying the case, killing three policeman in shootouts.
Police had arrested the detainee with 11 other members of an Islamist cell linked to Al-Qaeda that had planned to murder Raed Juhi this week.
The Saddam trial has been plagued by problems since its inception, most notably over security. Two defence lawyers have already been murdered.
During the fourth session on Tuesday, witnesses testifying behind a beige screen with electronically distorted voices described being beaten, pistol whipped and given electric shocks by Iraqi intelligence agents.
Their testimony was repeatedly challenged by defence lawyers and Saddam himself, who took the opportunity to make an outburst against the United States, Israel and the whole court.
Saddam harangued the Kurdish presiding judge, Rizkar Mohammed Amin, accusing him of being taken in by American theatrics and chastising him for not being more concerned with the detention conditions of defendants, and derided witnesses as "collaborators".