Iraq's PM has vowed to put an end to attacks like Saturday's deadly truck bombing in Baghdad, which he blamed on followers of ex-leader Saddam Hussein.
Mr Maliki said the blast shocked the Iraqi people and the world
The vehicle blew up in the mainly Shia al-Sadriya district in central Baghdad, killing more than 130 people and injuring at least 300.
Washington said the attack was another atrocity aimed at innocent Iraqis.
Free nations should not stand by while mass murder was committed, the White House statement said.
In fresh violence, four policemen were killed in a roadside bomb attack in the capital on Sunday, police said.
At least three others were wounded in the blast in the al-Qasra area of the capital.
Prime Minister Nouri Maliki said Iraqi people and the rest of the world were shocked by Saturday's bombing.
"The Saddamists have returned to commit a new crime," Mr Maliki said, blaming supporters of former Iraqi leader Saddam Hussein.
"We reassure the population that we will put an end to these crimes," he added.
An Iraqi government spokesman, meanwhile, said that half of the attacks in Baghdad were carried out by extremists who came from Syria.
Speaking on al-Arabiyah television, Ali al-Dabbagh said "50% of terrorism enters Iraq from Syria, and we have evidence" to prove that, the Associated Press news agency reported.
"The Interior Ministry and the Ministry of State for National Security gave them [the Syrians] evidence about those who are conspiring and are sending car bombs. We gave them the numbers of their apartments and the buildings where they live," he said.
'Tonne of explosives'
Saturday's blast took place as people bought food ahead of a night-time curfew.
It shattered stalls and left a huge crater in the street.
Maj Gen Jihad al-Jaberi of the interior ministry told state television the truck was carrying one tonne of explosives that were detonated by a suicide bomber.
Nearby hospitals were overflowing with dead and injured
Rescuers pulled the dead and injured from the rubble of the blast and loaded them on to pick-up trucks to be ferried to hospital.
Wards and corridors overflowed at the nearby Ibn al-Nafis hospital, with relatives screaming for help for their loved ones.
The same market was hit by car bombs on 2 December that killed more than 50 people.
Another attack, on the Haraj market in Baghdad on 22 January, killed 88 people.
BBC world affairs correspondent Mike Wooldridge says the attacks on markets are widely seen as Sunni insurgent groups stepping up their activities before a new joint Iraqi-US security operation in Baghdad gets under way.
He says the fear is always that bombings like this will bring retaliation from Shia militias.
Iraq's most prominent Shia cleric, Ayatollah Ali al-Sistani, made a new appeal to all Iraqis to "reject sectarian violence".