BAGHDAD, Iraq (CNN) -- Shiite cleric Muqtada al-Sadr is establishing a new fighting force to battle U.S.-led troops in Iraq, he said in a letter read in Iraqi mosques Friday.

Al-Sadr's letter said that "the resistance will be exclusively conducted by only one group. This new group will be defined soon by me."

Al-Sadr's militia, the Mehdi Army, has a strong and ubiquitous presence in Shiite cities, towns and neighborhoods.

Sources familiar with al-Sadr said they believe he's trying to embrace what the U.S. calls "Special Groups" -- Iran-backed Shiite militants, including rogue Mehdi Army members, who have been fighting U.S.-led troops despite a cease-fire that the cleric declared in August.

The mainstream Mehdi Army has operated under the cease-fire, which dramatically reduced violence in Iraq. During that time, however, there has been fighting between U.S. and Iraqi troops and members of the Mehdi Army, with many of the battles this spring in the southern city of Basra and Baghdad's Sadr City neighborhood.

In the latest fighting overnight, American-led coalition forces killed five and arrested two Special Groups members near Hilla, south of Baghdad, the U.S. military said.

In the letter, al-Sadr said, "The weapons will be held exclusively by this new group, and they should be pointed exclusively at the occupier," adding that he will forbid the group "to target anyone else."

The letter added, "We will not stop resisting the occupation until liberation or martyrdom."

Al-Sadr is transitioning much of the rest of the Mehdi Army into a civilian movement dealing with "religious, social and cultural affairs," according to the letter.

That part of the Mehdi Army will not be involved in militancy but will "fight the Western ideology and liberate the minds from domination and globalization."

The letter said al-Sadr would disown anyone in the Mehdi Army who disobeys his new command. It was read at mosques affiliated with the cleric's movement.

Iraqi and Western intelligence sources have said that Iran, over the last few months, has pressured al-Sadr to promote its interests, which include getting the United States out of Iraq.

Al-Sadr's letter didn't explain why he decided to issue his command.

Meanwhile, Iraqi officials, frustrated by the lack of success in negotiations with the United States over a long-term security agreement, are contemplating a new tack -- pulling out of security talks and developing their own legislation that would dictate the shape of the American military presence in Iraq.

Haidar Abadi, an aide to Prime Minister Nuri al-Maliki, announced the move in remarks Thursday.

The talks have sparked concern among Iraqis that a bilateral security pact will compromise their country's sovereignty.

The United States has said it hopes to secure a status of forces agreement by the end of July. At present, a U.N. mandate governs the American military presence through the end of the year.

CNN's Michael Ware, Saad Abedine and Yousif Bassil contributed to this report.

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