Bryan Whitman, a Pentagon spokesman, said: "Any time someone is determined to kill innocent civilians, and kill themselves to do it, that's a hard attack to stop."
On Thursday, the violence continued as a suicide car bomber rammed into a fuel truck, killing 11 injuring 21 in southern Baghdad.
Three people were also killed overnight when mortars were fired at a mainly Shia part of southern Baghdad.
Admiral William Fallon, the new commander of US forces in the Middle East, told US politicians in Washington on Wednesday that the surge in troops to Baghdad had significantly reduced the number of sectarian murders.
"The biggest concern I have are the periodic big bangs which are really troublesome because of the potential for retaliation and retribution," he said.
Co-ordinated blasts
US defence officials blamed al-Qaeda-linked groups for carrying out Wednesday's attacks.
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"The chances of success [in Iraq] are essentially zero because the Iraqi people have no voice"
Non Sequitur, Cadiz, Spain
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Major-General William Caldwell, a US military spokesman, said: "Initial indications based on intelligence sources show that it was linked to al-Qaeda."
Nuri al-Maliki, the Iraqi prime minister, blamed the attacks on "infidels and Sunni extremist vampires".
He has ordered the arrest of the Iraqi army commander in charge of security in Sadriya for failing to secure the area.
Ahmed Hameed, a shopkeeper in Sadriya, said: "The street was transformed into a swimming pool of blood."
The apparently co-ordinated attacks - there were several within a short space of time - occurred hours after al-Maliki said Iraqis would take security control of the whole country from foreign forces by the end of the year.
Ordinary Iraqis, however, demanded a solution to the daily bloodshed.
On Thursday at the site of the bombing in Sadriya, one man, who gave his name only as Ibrahim, said: "We want a solution to these massacres. What did those poor people do? Who are the targets? Woman and children?"
Spectacular attacks
The US defence department has spent billions of dollars to come up with ways to protect US troops from suicide bombings, car bombings and roadside bombings, with little to show for it.
The crackdown began of February 14 and has meant thousands of extra US and Iraqi troops being deployed in Baghdad.
The past week has seen several spectacular attacks in the capital, including a suicide bombing inside parliament and a powerful blast that collapsed a landmark bridge across the Tigris river.
The number of bodies dumped in the streets of Baghdad also has risen significantly.
"This is a very challenging situation where we're chasing a rabbit that runs pretty quickly," Fallon said.