Ziyad al-Samarrai, an Iraqi journalist speaking to Aljazeera from Baghdad, said a loud explosion rocked the base where US forces were present near al-Muthanna airport on Wednesday.
An Interior Ministry security official, Iyad Hadi al-Maliki, said 10 people were killed and 30 were wounded.
US and Iraqi forces reportedly have cordoned off the area near the airport.
Al-Samarrai said the bomber blew up his car as an Iraqi patrol, accompanied by Iraqi army recruits, was entering the base.
A similar bomb targeted the base a month ago.
"There were two soldiers lifted up into the air and knocked across the street"
Husain Muhammad, witness
Husain Muhammad, a witness who said he was 20 metres from the car that exploded, said: "It was a white Toyota Corolla trying to get inside the Iraqi army base. As he arrived, he (the driver) blew himself up."
"There were two soldiers lifted up into the air and knocked across the street," he added.
The explosion could be heard across the city, and a plume of black smoke billowed into the air. Flames leapt from two vehicles set ablaze.
A concrete wall protected some people from the blast, Muhammad said.
Residents said Iraqi security forces opened fire after the incident. It was not known what they were aiming at.
Al-Muthanna airport is home to
US and Iraqi military bases
US and Iraqi troops blocked roads and sealed off the area after the attack, preventing people from entering. Helicopters hovered overhead.
US Staff Sergeant Michelle Dodge of the 3rd Infantry Division, which is responsible for overall security in Baghdad, said: "There was an explosion but we are still investigating what the source of it was."
Al-Muthanna airport, no longer used for flights, is home to US and Iraqi military bases.
Another blast an hour later sent a thick cloud of smoke into the sky in southern Baghdad.
Al-Samarrai said the second explosion in Baghdad was not caused by a car bomb.
It was a series of explosions, caused by mortar shells, targeting an Iraqi National Guard camp in al-Dura neighbourhood south of Baghdad, he said.
US and Iraqi forces sealed off roads and ambulances were seen rushing to the site, which frequently is attacked by mortars.
An army officer said three soldiers were killed and six were wounded.
Also on Wednesday, doctors at Baghdad's Yarmuk hospital refused to treat wounded troops after doctors said their colleagues had been beaten up by soldiers.
"It all began when an officer told a doctor to stop using his mobile phone," Dr Amir Fadil said. "He replied that he had the right to make calls. I tried to intervene and a soldier hit me.
"I tried to intervene and a soldier hit me"
Amir Fadil, doctor at
Baghdad's Yarmuk hospital
"Another doctor was subjected to the same treatment and soldiers insulted a female colleague because she protested," he said, adding that the doctors held a meeting and decided to stop treating wounded soldiers.