His message was delivered by Shiite clerics during prayer services in mosques dominated by his followers.
"According to an order by Sayyid Muqtada, activities of the Mahdi Army will be suspended ... for another six-month period," al-Sadr aide Hazim al-Aaraji said during his sermon at the Kazimiyah mosque in Baghdad, according to the Associated Press.
Al-Sadr's decision to halt the activities of his powerful militia for up to six months last August was one of three critical steps widely credited with bringing the Iraqi death toll down more than 60 percent in recent months. Other factors include U.S. troop reinforcements and the move by American-backed Sunni Arab fighters to switch allegiances and start working against al-Qaida in Iraq.
Al-Sadr has said he needs time to reorganize his militia and the announcement was widely seen as a bid to bolster his image as a major player in Iraqi politics as Shiite leaders jockey for power ahead of an anticipated U.S. withdrawal.
Sheik Sadiq al-Essawi read the instructions in the Mahdi Army's Baghdad stronghold of Sadr City after opening the envelope that was sent to Sadrist clerics Thursday.
The U.S. military said the decision would allow American and Iraqi troops to focus "more intensively" on the fight against al-Qaida in Iraq.
"This extension of his August 2007 pledge of honor to halt attacks is an important commitment that can broadly contribute to further improvements in security for all Iraqi citizens," the military said in a statement, according to the AP. "It will also foster a better opportunity for national reconciliation."
The military command added that it was open to dialogue with the Sadrists and promised to treat members of the militia who honor the pledge "with respect and restraint" while cracking down on "criminals who violate the law and dishonor the commitment made by al-Sayyid Muqtada."
American troops have continued to raid Shiite groups alleged to be supported and trained by Iran and splintered off from al-Sadr's militia. That has angered some followers of al-Sadr, who also are frustrated with the Iraqi government. They had reportedly argued to end the cease-fire.
Meanwhile, Turkish troops crossed into northern Iraq hunting for Kurdish PKK rebels, the military said Friday.
Turkish TV said 3,000 to 10,000 soldiers entered Iraq, but Iraq's foreign minister and a senior military official with coalition forces based in Baghdad told Reuters only a few hundred troops were involved.
The United States urged Turkey to limit the incursions while the United Nations called on Turkey to respect Iraq's border.
"We were notified and we urged the Turkish government to limit their operations to precise targeting of the PKK, to limit the scope and duration of their operations," White House spokesman Scott Stanzel said, according to Reuters.
U.N. Secretary-General Ban Ki-moon called on Turkey Friday to respect its border with Iraq and urged the Kurdish PKK to end incursions into Iraq, the news agency reported.
"While conscious of Turkey's concerns, [he] reiterates his appeal for utmost restraint, and for respect of the international borders between Iraq and Turkey," a U.N. spokesman said in a statement.
Turkey has said it is allowed under international law to strike PKK rebels who shelter in northern Iraq and have mounted attacks inside Turkey that have killed scores of troops. Turkey says some 3,000 PKK rebels are based in Iraq.
Ankara blames the PKK for the deaths of nearly 40,000 people since it began an armed struggle for a Kurdish homeland in southeast Turkey in 1984. Washington and the EU, like Turkey, classify the PKK as a terrorist organization.