The anti-American religious leader, who heads the Mahdi Army militia, was first reported by the Americans to be absent from Iraq on Feb. 13, when the latest U.S.-Iraq security drive opened in Baghdad.
"He's a very significant part of this political process. We do continue to track his whereabouts," Maj. Gen. William B. Caldwell said at a briefing to mark the end of the first month of the security drive.
Iraqi officials with ties to al-Sadr have denied the militia leader fled Iraq.
The spokesman expressed particular concern about a spike last week in the number of what he called "high-profile" car bombings.
"If the high-profile car bombs can be stopped or brought down to a much lower level, we'll just see an incredible difference in the city overall. Murders and executions have come down by over 50 percent. ... But the high-profile car bombs is the one we're really focused on because that's what will start that whole cycle of violence again," Caldwell said.
The Mahdi Army has melted away and not confronted U.S. forces as American and Iraqi troops launched the third crackdown on sectarian violence in the capital in less than a year.
There was great concern the operation would force an all-out showdown with al-Sadr's forces in their Sadr City stronghold in eastern Baghdad, but that has not materialized.
As a result of the diminished presence of the Mahdi Army, the daily death toll of execution-style murders has dropped significantly.
That is part of a group of statistics that prompted Caldwell and U.S. ChargDe d'Affaires Daniel Speckhard to issue cautiously optimistic evaluations of the first month of the crackdown.
"By the indicators that the government of Iraq has, it has been extremely positive. But I would caution everybody about patience, about diligence. This is going to take many months, not weeks, but the indicators are all very positive right now," Caldwell said.
Meanwhile suicide bombers struck a market in northern Iraq and an Iraqi military checkpoint in Baghdad on Wednesday, killing at least 10 people, while an Iraqi general warned extremists that they will be "smashed under the foot of the Iraqi people" if they resist efforts to end the violence in the country.
In the worst attack, a man wearing an explosives belt strolled into an outdoor market in Tuz Khormato, 130 miles north of Baghdad, and blew himself up.
The blast occurred just before noon as the market was crowded with shoppers in the city, which has a mixed population with a slight majority of Turkomen. At least eight people were killed and 25 were wounded, police said.
Northern Iraq has seen a recent rise in violence that many blame on insurgents fleeing a security crackdown in the capital that began a month ago.
In western Baghdad, meanwhile, a suicide car bomber slammed into an Iraqi army checkpoint in the Sunni neighborhood of Yarmouk, killing two civilians and wounding four others, police said.
In other developments:
The commander of the Baghdad security plan, Lt. Gen. Abboud Qanbar, warned Wednesday that all terrorists and outlaws "will be smashed with the foot of the Iraqi people" unless they reconsider their "position and return to logic before it's too late."
Qanbar also sought to reassure the capital's residents that the military is not discriminating in the crackdown, despite complaints by Sunnis that their neighborhoods have been unfairly targeted by the Shiite-dominated government.
He said the effort had made headway in ending the sectarian violence that surged following the Feb. 22, 2006, bombing of a Shiite mosque in Samarra.
"We've overcome the terrorist acts, militant groups, criminal gangs, sectarian killings and displacement," he said at a press conference in the heavily fortified Green Zone. "We've passed to the other side and members of our armed forces are hunting down the terrorists and criminal gangs on the ground and in their dens."
"But we have to expect more obstacles and terrorist acts," he said. "These acts will not end immediately, but we will go forward with our operations until we annihilate the terrorism."
The U.S. military also has stepped up its presence with more than 20,000 extra American troops sent to Baghdad and surrounding areas as part of the security bid, which many have warned is a last chance to quell the violence.