Radical Shia cleric Moqtada Sadr has left Iraq for Iran, the senior US military spokesman in Baghdad says.
Moqtada Sadr leads a key bloc in the ruling Shia Alliance
Gen William Caldwell said Moqtada Sadr left some time last month.
However the cleric's aides strongly denied he had left. One Iraq government official also said Moqtada Sadr was in the holy city of Najaf on Tuesday.
Moqtada Sadr's Mehdi Army is one of the key forces in Baghdad but he has vowed not to interfere with the new government crackdown on militias.
Gen Caldwell's comments came as Iraqi and US forces implemented a new security "surge" in Baghdad, temporarily closing the borders with Syria and Iran, and extending the curfew in the capital.
Gen Caldwell said Moqtada Sadr had been tracked very closely.
"We will acknowledge that he is not in the country and all indications are in fact that he is in Iran," the general said.
"The reason why he is not in Iraq is not something I can discuss."
The new security "surge" is under way in Baghdad
Another US military source said media reports that Moqtada Sadr had been driven to Tehran two or three weeks ago were probably correct.
The US has identified the Mehdi Army as one of the biggest threats to law and order in the capital.
However Moqtada Sadr remains a key power broker, with his bloc holding about a quarter of the parliamentary seats of the ruling Shia Alliance of Prime Minister Nouri al-Maliki.
The head of the Sadr parliamentary bloc, Nassar al-Rubaei, denied the cleric had left: "He is now in Iraq. We fully support this security plan. It would make no sense for our leadership to escape it."
Some Sadr aides said he had kept a low profile for "security reasons", another that the Iran information was false and was probably put out deliberately to disguise the cleric's movements.
An Iraqi government official told Associated Press Moqtada Sadr had received a government delegation in Najaf, 160km (100 miles) south of Baghdad, on Tuesday night.
An Iranian official also denied the cleric had arrived there, the country's Irna news agency reported.
The US has blamed the Mehdi Army for much of the sectarian strife that has torn Iraq since the bombing of a key Shia shrine a year ago.
Moqtada Sadr, thought to be in his early 30s, is a youthful leader in a society that considers age and experience essential to religious authority.
He mixes Iraqi nationalism and Shia radicalism, making him a figurehead for many of Iraq's poor Shia Muslims.
Iraq's new security drive - Operation Law and Order - is now under way and includes:
- Closure of crossing points to Iran and Syria. Three to Syria and four to Iran will reopen after 72 hours but others will close indefinitely
- Weapons permits suspended in Baghdad to all but Iraqi and US forces and registered security firms
- The city's nightly curfew extended by an hour
- Increased stop and search powers in the capital.