Story Highlights• NEW: Largest security sweep of new Baghdad operation began Sunday
• In Basra, 30 captives, including women, children, possibly tortured, abused
• UK military: Detainees were in custody of Iraqi Intelligence Agency
• Iraq PM al-Maliki wants probe of "illegal, irresponsible" UK-Iraqi raid
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BAGHDAD, Iraq (CNN) -- Coalition forces began a massive "clearing operation" in Baghdad's dangerous Sadr City neighborhood Sunday, while in the southern city of Basra, the British military said a raid on an Iraqi intelligence office uncovered prisoners with signs of torture.
The operation in the capital is the largest security sweep of the mostly Shiite neighborhood since an Iraq-led security plan launched on February 14, according to the U.S. command in Baghdad.
The more than 1,000 U.S. and Iraqi troops who took part in the operation Sunday met no resistance, according to the U.S. military. Sadr City is a base for the Mehdi Army, militia loyal to anti-American Shiite cleric Muqtada al-Sadr.
"During operations today, local residents were receptive and cooperative with coalition and Iraqi forces," said spokesman Lt. Col. Scott R. Bleichwehl in a written statement. "The operation is designed to set secure conditions for the citizens of Sadr City."
In the coming days, after the clearing operation is completed, the first joint U.S.-Iraqi forces security stations are expected to be set up , as part of the new Baghdad security strategy.
Lt. Gen. Raymond Odierno, commander of the multinational corps in Iraq, said he expects it will be "a minimum of six to nine months" before Iraqi forces will be capable of maintaining order in Baghdad.
About 600 U.S. troops and 550 Iraqi soldiers and police took part in Sunday's operation in the eastern Baghdad district, the military said.
30 prisoners found
In Basra on Sunday, Iraqi and British troops discovered about 30 prisoners -- including a woman and children -- showing signs of torture at an Iraqi Intelligence Agency office, according to the British military.
Led by the Iraqis, the forces launched "an arrest and detention operation" for wanted terrorists at two successive locations, British Lt. Col. Kevin Stratfordright told CNN.
After capturing one suspected terrorist at the first location, the troops "were directed to the Iraqi Intelligence Agency -- a regional office in Basra -- where they suspected another target individual was located," he said.
At the intelligence office, according to a British military news release, the troops discovered "around 30 prisoners, including a woman and two children, who were being held and many of whom showed signs of torture and abuse."
But although Stratfordright told CNN the detainees had been in the custody of the Iraqi Intelligence Agency, he would not confirm the initial reports of torture.
"There was some suggestion by those forces on site that some of the detainees showed signs of abuse," Stratfordright told CNN. "At least one person on site used the word 'torture' in describing the situation on location. I can't confirm whether the word torture is accurate or not."
Neither is it clear who directed the troops to the intelligence office. But according to a British military statement, after coalition forces left, the prisoners escaped.
"It is unclear exactly how this occurred, though we can confirm that the Iraqi forces did not release the prisoners; nor was their release the intention of the operation," said the statement.
Iraqi Prime Minister Nuri al-Maliki issued a statement calling for an "urgent investigation" into who authorized the raid of the office -- but said nothing about how the prisoners came to be there.
Referring to the raid, the statement stressed "the need to punish those who carried out this illegal and irresponsible action."
CNN's Mohammed Tawfeeq and Jennifer Deaton contributed to this report.
An Iraqi soldier stands guard in Baghdad Saturday, a day before coalition forces launched a sweep in Sadr City.