Meanwhile, thousands of soldiers continued their search for three comrades abducted in a May 12 ambush south of Baghdad. Four other U.S. soldiers and one Iraqi were killed.
Over the weekend, the top U.S. commander in Iraq Gen. David Petraeus raised hopes about the search, reports CBS News correspondent Mark Strassman.
He told the Army Times he believed at least two of the missing soldiers were probably still alive, but that it couldn't be confirmed.
Petraeus gave no details or proof.
The commander also confirmed that the military knows who abducted the troops; an al Qaeda-connected group. But so far no one's been able to find the kidnappers, or their victims.
Former Marine Captain and author Nathaniel Fick said on CBS' Early Show that the intense danger and added human strain faced by the thousands of troops searching the area south of Baghdad, the search would likely go on until the missing soldiers are found.
"The human fatigue factor certainly has to be considered, but I think they have to keep searching. And I think they will," Fick told Early Show co-anchor Hannah Storm.
The minibus attack underscored the sectarian violence and instability that continues to plague Diyala province north of Baghdad despite the three-month-old security crackdown in Baghdad and the surrounding areas.
The bus, which left the town of Khalis, was driving near the violence-wracked city of Baqouba, 35 miles northeast of Baghdad, about 11:30 a.m. when it was ambushed outside the town of Hibhib, police said.
The stepped-up U.S. and Iraqi patrols of the capital during the crackdown have left the troops more vulnerable to attack by insurgents, military officials say.
The U.S. military reported Sunday that six U.S. soldiers on patrol in Baghdad were killed in a roadside bombing along with their interpreter on Saturday. A seventh soldier died in a blast Saturday in Diwaniya, a mostly Shiite city 80 miles south of the capital, where radical Shiite militias operate.
Those deaths brought the number of American troops killed in Iraq since Friday to at least 15 — eight of them in Baghdad. So far, at least 71 U.S. forces have died in Iraq this month — most of them from bombs.
In other developments:
In recent months, U.S. officials also have stepped up pressure on Iraq's religiously and ethnically based parties to reach agreements on a range of political and economic initiatives to encourage national reconciliation and bring an end to the fighting.
Progress in meeting those benchmarks is considered crucial to continued U.S. support for Prime Minister Nouri al-Maliki's government at a time when Democrats in Congress are pressing for an end to the war. Those benchmarks include enactment of a new law to manage the country's vast oil wealth and distribute revenues among the various groups.
But prospects for quick approval received a setback Sunday when the country's Sunni vice president told reporters in Jordan that the proposed legislation gives too many concessions to foreign oil companies.
"We disagree with the production sharing agreement," Tariq al-Hashemi told reporters on the sidelines of the Jordan conference. "We want foreign oil companies, and we have to lure them into Iraq to learn from their expertise and acquire their technology, but we shouldn't give them big privileges."
The bill also faces opposition from the Kurds, who have demanded greater control of oil fields in Kurdish areas. Kurdish parties control 58 of the 275 parliament seats.
Iraq's Cabinet signed off on the oil bill in February and sent it to parliament, a move that the Bush administration hailed as a major sign of political progress in Iraq. But parliament has yet to consider the legislation.
Al-Hashemi is among three leaders of a Sunni bloc that controls 44 seats. Together, the Kurds and the Sunnis have enough legislative muscle to delay passage of the measure, which is likely to draw opposition from some Shiite lawmakers, too.