"At 7:15pm (16:15GMT) a bomb exploded as a five-vehicle US convoy passed through Khalidiyah", Yasser Hamdan, 42, told AFP.
The blast was followed by a rocket-propelled grenade attack and a firefight, said witnesses.
Two of the wounded are said to be in serious condition. Two US military vehicles were completely destroyed.
Day of attacks
Earlier, five US soldiers were wounded when a US military Black Hawk helicopter was shot out of the sky in a rocket-propelled grenade (RPG) attack.
The chopper crash-landed near the town of Tikrit, former President Saddam Hussein's birthplace, an army spokesperson who declined to be named told AFP.
A doctor said one of those injured was suffering from shrapnel wounds in both the arms and legs.
“A helicopter did go down in Tikrit. I do not know if it was a controlled landing. Five people were wounded. It was an RPG attack,” the spokesperson said.
The chopper was seen by an AFP photographer trailing smoke minutes after a loud explosion. The sound of small arms fire was also heard from the direction of the emergency landing, the French news agency reported.
US assistant defence secretary, Paul Wolfowitz, on a tour of Iraq, visited Tikrit only hours before the attack on Saturday.
Elsewhere, the US military confirmed three soldiers were wounded in a bomb attack in Baghdad.
Captain Ty Wilson of the 1st Armoured Division said a makeshift bomb, possibly a mortar round detonated by remote control, exploded at dawn on Saturday, on an exit ramp off a highway in the southwest of the capital.
“A helicopter did go down in Tikrit. I do not know if it was a controlled landing. Five people were wounded”
US military spokesman
Wilson said the bomb, placed beside a steel guard rail under a tree, had hit a Humvee military vehicle.
In another incident, eleven members of a martial arts team were wounded, three of them seriously, when unknown gunmen opened fire on their bus near the northern oil town of Kirkuk.
The Kurdish Taekwondo team was heading back to Kirkuk from a Baghdad tournament when they came under attack.
Doctors said that three of the team members were in serious condition and had been operated on five times.
Several other attacks were also carried out across Iraq in the past 24 hours, targeting either the US forces or Iraqi police, Aktham Sulaiman, Aljazeera's correspondent in Iraq, reported
“An Iraqi police station came under a series of attacks in the city of Mosul,” he added.
According to Sulaima “a 20kg explosive device was found in Baghdad’s Press Association building”.