A U.S. soldier at the scene said several troops were wounded or killed when the blast hit the lead vehicle.
Footage from Associated Press Television News showed blood inside a slightly damaged Humvee and a flak vest laying in the road.
Word of the blast came as three Turkish hostages were released by militants in Iraq, the Turkish foreign minister said.
"Our citizens have been released," Foreign Minister Abdullah Gul told state television. "We've struggled a lot for their release."
In other recent developments:
Word of the release of the Turkish men first surfaced in a report on Al-Jazeera television that said an extremist group responsible for beheading two other foreign hostages had announced it was releasing the three Turk hostages "for the sake of their Muslim brothers."
The Arab satellite station broadcast a videotape showing the three, believed to have been contractors, kneeling in front of three members of Abu Musab al-Zarqawi's Tawhid and Jihad movement, as one of the militants read a statement.
"For the sake of you, our brothers, and Muslims of the people of Turkey ... we will release these hostages and send them safely home," the statement said.
Two other Turkish hostages held by militants in Iraq were allowed to call their families to say they would be freed within a week after their company agreed to kidnappers' demands to stop working as a contractor for the U.S. military in Iraq, CNN-Turk television reported.
The fathers of the two hostages said in phone interviews Tuesday with the private network that the two hostages — Soner Sercali, an air conditioning repairman, and his co-worker Murat Kizil — were in good health. They have been reported missing since June 1.
Sercali's father Feridun said their company, Kayteks, had agreed to stop doing business in Iraq.
Neither the company nor the families could be reached for comment Tuesday.
The Turkish news agency Ihlas on Monday released a photograph of the two hostages showing them squatting in front of five masked men armed with machine guns and rocket-propelled grenades. The agency said its photographer took the picture at an unspecified location in Iraq.
Thousands of Turks work as truck drivers or contractors in Iraq. The kidnapped Turks were accused of working for the U.S. occupiers.