A senior Iraqi official has said half of all "insurgent attacks" in Baghdad are carried out by militants from Syria.
Ali al-Dabbagh said the Iraqi government has provided Damascus with evidence to back up this claim.
It comes after the deadliest single bombing since the US-led invasion of 2003 killed 130 people in Baghdad.
Iraqi Prime Minister Nouri Maliki has vowed to put an end to such attacks, which he blamed on followers of former Iraqi leader Saddam Hussein.
In fresh violence, four policemen were killed in a roadside bomb attack in Baghdad on Sunday, police said.
At least three others were wounded in the blast in the al-Qasra area of the capital.
'Car bombs'
Saturday's suicide truck bombing tore through a market in al-Sadriya district of Baghdad, causing widespread devastation.
More than 300 people were injured in the attack - the fifth major bombing in less than a month targeting mainly Shia areas of the capital and in Hilla, a town to the south.
Speaking on al-Arabiyah television, Mr al-Dabbagh said many of the fighters emanated from neighboring Syria.
"Fifty per cent 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.
The Iraqi government, and Washington, have long accused Syria of failing to prevent militants crossing the border.
The BBC's world affairs correspondent in Baghdad, Mike Wooldridge, says the Syrians maintain they have taken action and it is up to the Americans and the Iraqis to do more.
Prime Minister Maliki said the scale of Saturday's attack had shocked Iraqis and the rest of the world.
"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.
Our correspondent says the relentlessness of these attacks in places where they are clearly intended to cause the biggest number of casualties underlines the challenge before the Iraqis and Americans as they build up their forces for their new security operation in Baghdad.
He says they have to prove to an often skeptical public here that the threat from bombers and gunmen can be significantly lowered.
Photo caption
Scene of Baghdad bombing