Radical anti-US cleric Moqtada al-Sadr warned that any US training mission in Iraq after 2011 would amount to an occupation force and need to be opposed by "military means," in a letter seen on Sunday.
The letter, released on Saturday in the Shiite cleric's base in the shrine city of Najaf, came after Baghdad agreed to open talks with Washington for a training mission post-2011, when all US troops are due to have left.
"We will treat anyone who stays in Iraq as an oppressive occupier that should be resisted through military means," Sadr, who divides his time between Iran and Najaf, said in a letter released by his office.
"The government that agrees to their stay, even if it is for training, is a weak government."
His movement issued a code of conduct to followers last month stating they must consider "as enemies only the United States, Britain and Israel, and take into account that military resistance should be conducted by specialists."
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Sadr's remarks come after Iraqi political leaders agreed on Wednesday to start negotiations with Washington on a US military mission to train Iraqi security forces.
Unresolved issues remain over the size of the force, the duration of its stay, and whether its members would enjoy immunity from prosecution.
About 47,000 US troops are still stationed in Iraq, all of whom must leave by the end of the year under the terms of a 2008 bilateral security pact, which would remain in force if a training deal is not agreed.
US and Iraqi military officials assess Iraq's security forces capable of maintaining internal security, but say the country is lacking in terms of capacity to defend its borders, airspace and territorial waters.
Sadr's movement has 40 deputies in parliament and seven ministers in Prime Minister Nuri al-Maliki's national unity government.
And before it was disbanded in 2008, Sadr's Mahdi Army numbered some 60,000 fighters with fierce loyalty to the cleric. It fought bloody battles with the US army in the years following the 2003 invasion which ousted Saddam Hussein.
On July 10, Sadr said he would not revive the Mahdi Army, complaining it had been infested with "criminals."