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NAJAF, Iraq, April 7 (Reuters) - Iraqi Shi'ite cleric Moqtada al-Sadr will disband his Mehdi Army militia if top Shi'ite clerics including Grand Ayatollah Ali al-Sistani order him to do so, a senior Sadr aide told Reuters on Monday.

Aide Hassan Zargani told Reuters that Sadr had told his representatives in both the holy Iraqi city of Najaf and also the Iranian city of Qom to ask top Shi'ite religious leaders for advice on whether to dissolve the militia.

"If they order the Mehdi Army to disband, Moqtada al-Sadr and the Sadr movement will obey the orders of the religious leaders," said Zargani, who was speaking from Iran.

The development came after Prime Minister Nuri al-Maliki raised the stakes in a showdown with Sadr's millions of followers by saying they would be barred from elections later this year unless the Mehdi Army was dissolved.

"Moqtada al-Sadr has ordered his office in Najaf and Qom to form a delegation to visit Sistani in Najaf and (other leaders) in Qom to discuss the disbanding of the Mehdi Army," said Zargani. The ageing Sistani is the highest Shi'ite religious authority in Iraq and is revered by millions. He lives in Najaf.

Maliki launched a crackdown on the Mehdi Army in the southern city of Basra late last month, triggering an uprising across Baghdad and Shi'ite southern Iraq.

(Reporting by Khaled Farhan, Writing by Dean Yates, Editing by Peter Graff)

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