Bombs killed at least 40 people on Tuesday, half of them Iraqi soldiers, in the latest blow to the new government's efforts to inspire confidence in the country's security forces.
The most dramatic blast was a roadside bomb attack on a bus filled with soldiers on a road between Tikrit and Baiji north of Baghdad. At least 20 of them were killed.
A source at the joint Iraqi-US military coordination centre in Tikrit said the toll could rise.
"The bodies are being removed from the bus," he said.
In Baghdad, a suicide bomber in a car targeted soldiers collecting their salaries from a bank, killing at least 10 people, including an elderly women.
The attack took place at the same spot in the relatively stable district of Karrada where a car bomb and mortars killed at least 27 people last week.
An agency reporter saw bodies that were charred or ripped apart by the blast lying on the pavement on a street lined with shops.
One boy about 12-years old stood in the street sobbing and tearing his shirt after seeing his dead mother.
"My mother, my mother, my mother," he screamed, as people held him back from reaching her corpse.
A woman nearby searched frantically for her son.
"Has anyone seen Ali? Has anyone seen Ali," she said before collapsing on the ground, helpless.
Two months after being sworn in, Iraqi Prime Minister Nuri al-Maliki has yet to prove he can put an end to such scenes of carnage and ease sectarian violence, which has raised fears of civil war.
So far, he has presented a 24-point reconciliation plan that is long on promises and short on details and imposed a security crackdown in Baghdad that has failed to ease bloodshed.
The United States plans to boost its troop levels in Baghdad in a bid to improve security but long-term stability, and an American withdrawal, depends on the performance of Iraqi forces.
Iraqi security forces were also targeted in the town of Muqdadiya, 90 km (50 miles) northeast of the capital.
A car bomb exploded as a police patrol passed by in front of a hospital, killing at least seven people and wounding eight, police said.
In the northern oil city of Kirkuk, a roadside bomb killed two police and wounded a third as they conducted a patrol.
A roadside bomb killed one person in the Zayouna district of Baghdad, police said