Three Sunni Muslim mosques were attacked and burned south of Baghdad on Thursday, Iraqi police said, in apparent reprisal attacks after suspected Al-Qaeda militants blew up the minarets of a revered Shi'ite shrine.
Tens of thousands of Iraqi and US soldiers were on the streets of Baghdad and other cities enforcing curfews imposed after Wednesday's bombing at Samarra's al-Askari mosque toppled its two golden minarets.
An attack on the same mosque in February 2006 unleashed waves of sectarian violence in which tens of thousands of people were killed, tipping Iraq close to all-out civil war between majority Shi'ite Muslims and minority Sunni Arabs.
The latest Samarra attack, condemned by US President George W Bush and other world leaders, immediately raised fears of similar retaliatory violence. Prime Minister Nuri al-Maliki ordered an extra brigade of Iraqi security forces to Samarra.
"We want to make sure we don't see a repeat of what happened after February 2006," US military spokesman Lieutenant-Colonel Christopher Garver said.
The streets of Samarra were quiet on Thursday, but there was a grim mood as Iraqi soldiers fanned out around the mainly Sunni city. Residents said military snipers could be seen on rooftops.
Others said US soldiers were seen in Samarra but Garver said they were only helping Iraqis with transport and other specialised roles. "We are not providing troops on the ground for security reasons. This is an Iraqi-run operation," he said.
While Samarra was quiet, violence spilled over in Baghdad and other towns and cities to the south.
Police said unidentified gunmen on Thursday attacked the al-Mustafa and Huteen mosques in the town of Iskandariya, where the Sunni Grand Mosque was destroyed on Wednesday. The al-Bashir mosque in nearby Mahaweel was also attacked.
A woman and child were wounded in the attacks, police said.
In the southern Shi'ite cities of Najaf and Basra and the sprawling Sadr City slum in Baghdad, supporters of anti-American cleric Moqtada al-Sadr took to the streets in noisy but peaceful protests over the Samarra bombing, residents said.
US commanders have said all American troop reinforcements will be in place by Friday as part of a major security crackdown in the capital. Some 28,000 extra US soldiers are being sent, most of them to Baghdad.
The crackdown is aimed at securing the capital so Maliki's Shi'ite-led government can reach political targets set by Washington aimed at promoting national reconciliation.
The crackdown was launched in mid-February and had early success in reducing the number of targeted sectarian murders between Shi'ites and Sunni Arabs, who were dominant under Saddam Hussein but now form the backbone of Iraq's bloody insurgency.
But civilian deaths rose by 29 per cent in May to almost 2,000, the highest level since the crackdown began.
US military deaths have also risen during the crackdown as thousands more troops fan out through the streets of Baghdad, becoming more visible targets.
A total of 126 were killed in May, the highest monthly total for two-and-a-half years and the third-highest since the invasion to topple Saddam began in March 2003.
The increased number of civilian and military deaths suggest General David Petraeus, commander of US forces in Iraq, and US ambassador Ryan Crocker will likely take a bleak message when they present a progress report to Washington in September.
Bush last month described that upcoming report as an "important moment" but White House officials have since sought to temper expectations of rapid results from the crackdown.