By Aseel Kami
Salah al-Ubaidi said the ceasefire, which expires later this month, should continue to be observed until militia members are told it is over or has been renewed.
Attacks across the country have fallen by 60 percent since June 2007 and a return to hostilities could seriously jeopardise those security gains.
He said Sadr had issued the statement in response to rumours that the ceasefire was about to come to an end.
Amid signs of growing restlessness, Iraqi police said Mehdi Army fighters had clashed with Iraqi and U.S. soldiers early on Thursday in Sadr City, the sprawling Shi'ite slum in northeast Baghdad which is one of Sadr's power bases.
A U.S. military spokesman said one person was killed and another was injured when U.S. and Iraqi soldiers conducted raids "targeting criminal elements".
Mehdi Army fighters had often been involved in fierce clashes with U.S. troops or Sunni Arab groups, and the Pentagon once described it as the greatest single threat to peace in Iraq -- a term now it now uses for Sunni Islamist al Qaeda.
Recent statements from within Sadr's camp have indicated growing unease about the truce, with members claiming they are being targeted by Iraqi security forces.
(Writing by Paul Tait; Editing by Catherine Evans)
