"Keep fighting even if you see me a prisoner or a martyr. God willing, you will be victorious," the anti-US occupation leader said in a statement on Wednesday.
Al-Sadr said he still wanted Iraq to remain united and thanked "those who tried to resolve the crisis peacefully".
Al-Sadr's Al-Mahdi army fighters have been battling US-led forces in central and southern Iraq, including the city of Najaf, which has come under bombardment, with the latest fighting lasting for more than a week. The casualty toll is in the hundreds and still mounting.
On the ground in Najaf, renewed clashes broke out on Wednesday after a relatively calm night, with US forces apparently trying out a new line of attack.
The official spokesman for al-Sadr's office in Najaf, Shaikh Ahmad al-Shibani, interviewed by Aljazeera, said Al-Mahdi fighters were resisting US helicopters, not ground forces, "due to the failure of their (the US) soldiers in direct military confrontation".
"Keep fighting even if you see me a prisoner or a martyr. God willing, you will be victorious"
Shia Muslim leader
"The fighting is concentrated in the cemetery north of Najaf," al-Shibani said.
"However, the cemetery is under the control of Al-Mahdi army and no US forces are based there."
Asked about the latest casualty figures, al-Shibani said that 20 Al-Mahdi army members had so far been killed and another 35 had been injured.
In Baghdad, Aljazeera has reported that a leader of the Badr Brigades has been assassinated by unknown attackers in the Mahmudiya area, south of the capital.