The Pentagon has defended its security crackdown in Baghdad after more than 140 people were killed in one attack near a market - the worst single blast in the capital since the US-led invasion in 2003.
"Frankly I would like to see faster progress," Gates said.
On Thursday, the violence continued as a suicide car bomber rammed into a fuel truck, killing 12 people and injuring 24 others in the Jadiriya district of Baghdad according to Al Jazeera.
Three people were also killed overnight when mortars were fired at a mainly Shia part of southern Baghdad.
Soldiers killed
Two British soldiers were killed by a roadside bomb in the southern province of Maysan on Thursday, the British military said, a day after it handed over the region to Iraqi security control.
On Wednesday, a US soldier was killed after his patrol was attacked in southwest Baghdad while two more soldiers were killed and another wounded when their vehicle was struck by a roadside bomb north of Baghdad, the US military reported.
The US military also reported that a US marine died in a non-hostile shooting incident during combat operations in western Anbar province on Monday, the US military said.
It said it was investigating the incident.
Spike in violence
Speaking to Al Jazeera on Thursday, Brigadier-General Bob Holmes from the US Central Command in Qatar said that the Baghdad security plan was still being implemented and that other developments needed to dovetail with an increased military presence.
He said: "Central Command have said you can't just expect a military solution to work by itself.
"It takes overarching diplomatic means, political instruments of power, economic development, societal, cultural all of these things have to come together with in this case the military and support as we support the Iraqis to give a stable base."
A day earlier, Admiral William Fallon, the new commander of US forces in the Middle East, told US politicians in Washington that the surge in troops to Baghdad had significantly reduced the number of sectarian murders.
He said: "The biggest concern I have are the periodic big bangs which are really troublesome because of the potential for retaliation and retribution."
Co-ordinated blasts
US defence officials blamed al-Qaeda-linked groups for carrying out Wednesday's attacks.
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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?"