Radical Iraqi Shia cleric Moqtada Sadr is currently in Iran, an adviser to the Iraqi prime minister has said.
Moqtada Sadr leads a key bloc in the ruling Shia Alliance
Sami al-Askari said the cleric would be back soon and had not fled the country in response to the new US-Iraqi security plan for Baghdad.
There have been conflicting reports about Mr Sadr's whereabouts - the US said he left for Iran last month but his aides said he was still in Iraq.
The cleric's Mehdi Army is one of the key forces in Baghdad
The US military has identified the Mehdi Army as one of the biggest threats to law and order in the capital.
However, Mr Sadr has vowed not to interfere with the new government crackdown on militias.
His bloc holds about a quarter of the parliamentary seats of the ruling Shia Alliance of Prime Minister Nouri Maliki.
Mr Sadr, thought to be in his early 30s, mixes Iraqi nationalism and Shia radicalism, making him a figurehead for many of Iraq's poor Shia Muslims.
In southern Iraq, British troops have set up checkpoints on the major routes into Basra in support of the new security plan.
Two nearby border crossings with Iran had been closed for 72 hours, in accordance with the Iraqi government's instructions, a British military spokeswoman said.
The Iraqi and US security crackdown which began on Wednesday, also includes the closure of crossing points to Syria.
Three crossing points to Syria and four to Iran will reopen after 72 hours but others will close indefinitely.