The group which freed the soldiers on Monday had threatened to kill the captives unless an aide to Shia leader Muqtada al-Sadr was released.
On Sunday, Aljazeera showed a video from the captors standing near a group of uniformed men it had seized, apparently from Iraq's National Guard.
The captors were demanding the release of al-Sadr aide Hazim al-Aaraji, a Shia cleric, within 48 hours or they would kill their captives.
In an interview with Aljazeera, a senior aide to al-Sadr, Shaikh Ali Smaisim earlier on Monday denounced the capture of the guardsmen and called for their release.
"I condemn this action despite the assassinations, arrests, raids, and abductions carried out by the 'occupation' forces against al-Sadr movement and all other national and political forces that oppose the 'occupier'," he said.
"There is a conspiracy being set up against all movements that oppose the 'occupation' forces"
Shaikh Ali Smaisim,
aide, Muqtada al-Sadr
"There is a conspiracy being set up against all movements that oppose the 'occupation' forces," Smaisim said. "Their members get arrested, assassinated or kidnapped," he added.
Al-Sadr supporters in Baghdad said US forces had arrested al-Aaraji in a raid in the Iraqi capital.
Al-Sadr's supporters have launched two uprisings this year in Najaf and fought battles with government and US-led forces in Baghdad.