The city, which is known for tomatoes and cattle, has suffered frequent insurgent attacks, despite sand barriers erected around it by US and Iraqi forces. But the situation had been calm in recent months and some displaced Shiite and Sunni families had started to return to their homes.

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Army troops moved into Sunni areas of the city Wednesday to stop the violence, confining police to their bases and enacting a curfew. Calm was restored by mid-afternoon, said Wathiq al-Hamdani, the provincial police chief, and his head of operations, Brig. Abdul-Karim al-Jibouri.

Hundreds of residents - including relatives of bombing victims - rallied later Wednesday, demanding the resignations of the police chief and the mayor, as well as the release of the detained Shiite policemen.

Meanwhile, US officials remained cautiously optimistic about a security crackdown that is entering its seventh week in Baghdad. The US has sent about 30,000 extra troops to assist in the operation, which many see as a last-ditch attempt to bring peace to Iraq.

"We are seeing preliminary signs of progress," US military spokesman Rear Adm. Mark Fox said Wednesday, but he cautioned that "there will be rough days ahead."

"Progress does not come without a cost," he said. "Like backing a rat into a corner, increasing pressure on the extremists by limiting their available resources and places to hide leads to desperate changes in tactics."

The new tactics included using chlorine, a highly toxic chemical, in at least eight bombings since Jan. 28. On Wednesday, suicide bombers detonated explosives on trucks carrying the chemical outside the Fallujah government center, about 40 miles west of Baghdad.

About 15 US and Iraqi security forces were wounded in the attack, although the bombers were shot and set off their explosives before reaching government buildings, the US military said.

At least 44 people were killed or found dead elsewhere in Iraq, including four people in two car bombings in Baghdad. A parked car bomb struck a market in the predominantly Shiite city of Mahaweel, south of Baghdad, killing at least four people.

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