The U.N. chief and Iraqi leader were holding a joint news conference when the explosion occurred. It was not immediately clear what caused the blast.
In other news, the U.S. military said coalition forces in Basra and Hillah have captured members of the Khazai network that it says was directly connected to the killing of five American troops in Karbala in January.
In that attack, insurgents wore U.S.-style military uniforms and passed through security at a command post in Karbala, killing one American soldier and kidnapping and killing four others.
In other developments, an Iraqi official said the government has been in talks with Sunni insurgent groups over the past months, in an effort to persuade them to disarm.
In interviews with Western news agencies, BBC and the Associated Press, Saad Yousif al-Muttalibi of Iraq's National Dialogue and Reconciliation Ministry declined to identify the groups, but said al-Qaida is not involved.
In other news, the U.S. military said coalition forces freed three hostages and detained 13 suspected terrorists in raids targeting al-Qaida.
The military also said three American troops were killed Wednesday in combat operations in Baghdad and al-Anbar province.
Some information for this report was provided by AFP and AP