Amara is known as a stronghold of Shia leader Muqtada al-Sadr and the impending crackdown comes as al-Sadr supporters said on Sunday that they would boycott provincial elections due in October.
The announcement by Sheikh Salah al-Obeidi, a spokesman for al-Sadr in the city of Najaf, is the latest blow to the embattled Iraqi political process.
"The Sadr group will not take part in the [provincial] elections as we did in the parliamentary election," al-Obeidi said.
"This is the decision as of now by Muqtada and the Sadrists. We want to avoid making the same mistakes of being part of the sectarian divisions."
Iraq is due to hold elections in its 18 provinces on October 1, a move intended to give more power to local provincial councils.
The provincial elections law was passed in February amid criticism that some aspects of it were in contradiction with the Iraqi constitution.
The elections are a benchmark set by Washington.
The al-Sadr group has 32 legislators in Iraq's 275-member parliament and its decision not to take part in the elections is seen as a step to consolidate its image as a nationalist and anti-American movement.
Al-Obeidi said the group will not directly participate in the elections but will support "independent" candidates.
The decision comes two days after al-Sadr decided to reform his al-Mahdi Army militia.
On Friday, al-Sadr said he planned to form a new wing of his movement specifically to battle US forces, allowing other members to focus on social issues.
In a statement issued to his nearly 60,000-strong militia, he said the fight against US troops will now be waged only by the new group, while other members will "take on a social and religious role".
"We will keep resisting the occupier until the liberation [of Iraq] or [our] martyrdom," he said.
Al-Sadr also urged others in his militia to lay down their weapons to work on building social, cultural and religious services among Iraq's Shia population.