The newly arrived include U.S. veterans who have tried to sign on with Iraqi Christian militias.
Officials called the move a response to Islamic State militants who have destroyed ancient artifacts in Mosul.
Securing the Islamic State stronghold is seen as an essential step to retaking Mosul.
WORLDVIEWS | A document circulated online also says Abu Bakr al-Baghdadi excelled in math.
Moqtada al-Sadr took the decision following reports of executions and mass killings by “brazen” militias.
Basra residents complain that the government ignores their needs while relying on the province’s oil revenue.
Politicians blame the government, which they say allows Shiite militias to act without recourse.
Iraqi security forces repelled the attack by eight would-be suicide bombers outside Ayn al-Asad base.
Militants came within two miles of the base, according to tribal fighters.
The course of true love never did run smooth, and in Basra, a city with a strong presence of religiously conservative Shiite militias, such a public display of affection inevitably risked a backlash.