Local authorities clamped a total curfew on the Shiite holy city of Najaf in central Iraq on Friday to thwart attacks a day after 73 people died from twin suicide bombings in nearby Hilla.
In Baghdad, the US military was checking reports that another of its helicopters had gone down near Taji, north of the Iraqi capital.
In Najaf, "the curfew imposed overnight has been extended and includes the entire region around Najaf including Kufa, northeast of the holy city," police said.
Najaf provincial spokesman Ahmad Duaibel added: "Authorities took this decision after receiving information that armed groups planned to launch attacks against the city."
On Sunday, Iraqi and US forces clashed with members of a Shiite sect north of Najaf, killing more than 250 "Soldiers of Heaven", wounding more than 200 and arresting almost 300 when fighting ended early on Monday, a defence ministry spokesman said on Tuesday.
Officials said earlier this week that the well-armed militia and its cult leader planned attacks on senior Shiite clerics in Najaf and were preparing to occupy holy sites there.
In Baghdad, US Lieutenant Colonel Josslyn Aberle told the agency the military was checking reports of another helicopter crash.
"We're looking into reports of a helicopter going down north of Baghdad at this time," she said.
It would be the fourth crash of a US military or civilian chopper since January 20.
An Iraqi army source had reported earlier that soldiers saw the US aircraft go down near a US air base in Taji.
Meanwhile, at least 86 people died in Iraq Thursday as security officials reported that bitter sectarian attacks had claimed the lives of nearly 2,000 civilians throughout the country in January.
Two suicide bombers killed 73 people and wounded 163 when they targeted the centre of Hilla, the predominantly Shiite capital of Babil province south of Baghdad, security and medical sources said.
Twenty-five of the wounded were in critical condition, hospital director Mohammed Diaa said.
Policemen tried to disarm the first bomber but could not stop twin blasts which struck a crowded market, police officer Kazem al-Shamari said.
The initial blast sent a panicked crowd fleeing in the direction of the second bomber, who then blew himself up, he added. More than 20 shops were destroyed in the attack.
In sparsely populated southern Iraq, a grave containing the remains of 200 to 250 people — probably Kurds — was discovered near the border with Saudi Arabia, a provincial official told the agency.
Amin Mohammed Amin said the grave was found west of the town of Salman, less than a kilometre from a former detention camp, thanks to reports from inhabitants of the mainly deserted region.
The grave extended over more than 200 square meters (2,000 square feet) and included the bodies of men, women and children in Kurdish clothing, he added.
A security source said Thursday that nearly 2,000 civilians were killed in January, mostly in sectarian violence, and added that the number of wounded was significantly higher than in December.
He said that "1,992 civilians were killed by violence in Iraq during the month of January," speaking on condition of anonymity and citing health ministry figures.
The number of wounded civilians in January was 1,941 — a sharp increase from 1,511 in December.
US President George W Bush's plan to send 21,500 troops to help stem the sectarian bloodshed could result in an increased US force of up to 48,000 troops, a report concluded Thursday.
The Congressional Budget Office said that when non-combat support troops were included, the real number of additional troops deployed in Iraq could reach 35,000-48,000.
General George C Casey, the outgoing US commander in Iraq, said Baghdad could be secured with fewer US troops than foreseen, and defended his past reluctance to call for reinforcements even as sectarian violence spread.