Abdel-Zahraa Othman, the current president of the U.S.-appointed Iraqi Governing Council, was one of nine people killed in the blast, Iraqi officials said, according to the Associated Press. The presidency of the Governing Council is seen as a largely symbolic role and all members of the council rotate through the post on a monthly basis.
A previously unknown group calling itself the Arab Resistance Movement, Al-Rashid Brigades, claimed responsibility for the attack in a statement posted on a Web site devoted to Iraqi issues.
The group said it would continue its "jihad" or holy war until Iraq and Palestine are liberated, the AP reported.
U.S. Brig. Gen. Mark Kimmitt said terrorist groups are trying to derail the democratization process in Iraq.
"Days like today convince us even more so that the transfer must stay on track," he said on CNN.
The new head of the governing council, Ghazi Mashal Ajil al-Yawer, said the council would "not deviate from the march to which he devoted his life -- the march of glory, happiness and freedom for our people ... the march toward building a democratic, federal, plural and unified Iraq."
"God willing, the criminal forces will be defeated despite all the pain they are causing to our people and their heroic leaders," he said, according to the AP.
Othman, also known as Izzadine Saleem, was a Shiite Muslim who led the Islamic Dawa Movement in the southern city of Basra. He was a writer, philosopher and political activist, and edited several newspapers and magazines.
Iraqi Deputy Foreign Minister Hamed al-Bayati said other council members near the checkpoint at the time escaped unharmed, according to Reuters.
More than a dozen vehicles were destroyed in the blast.
The assassination of Othman came only hours before the U.S. military confirmed the presence of a chemical weapon in Iraq. A roadside bomb containing sarin nerve gas exploded near a U.S. military convoy, officials said Monday. Two people were treated for "minor exposure," but no other serious injuries were reported.
The Iraqi Survey Group, a U.S. organization tasked with searching for weapons of mass destruction, confirmed that a 155-millimeter artillery round contained the nerve agent, said Kimmitt.
"The round was an old binary-type requiring the mixing of two chemical components in separate sections of the cell before the deadly agent is produced," he said, according to the AP. "The cell is designed to work after being fired from an artillery piece."
The round was rigged as an improvised explosive device but detonated before it could be defused, releasing some of the agent, Kimmitt said.
In 1995, Japan's Aum Shinrikyo cult released sarin gas in Tokyo's subway, killing 12 people and sickening thousands.
In southern Iraq, U.S.-led forces reportedly killed 51 guerillas, including 20 in an airstrike as insurgents were loading and unloading weapons from vehicles, the U.S. military said, reported Reuters.
Kimmitt said an additional 17 militiamen loyal to radical cleric Muqtada al-Sadr were killed near shrines in the holy city of Karbala, and 13 more were killed in other areas.