Story Highlights• Roadside bomb attack kills 9, including 5 U.S. soldiers
• Operation Phantom Thunder under way to crack down on extremists in Iraq
• Gen. Peter Pace: Success of offensive shouldn't be measured by levels of violence
• Suicide bomber rams truck packed with explosives into government building
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BAGHDAD, Iraq (CNN) -- Twelve U.S. troops have been killed in attacks in expanded operations against insurgents in Iraq during the past 48 hours, according to the U.S. military.
In the deadliest attack, a roadside bomb Thursday struck a military vehicle in northeastern Baghdad, killing five U.S. soldiers, three Iraqi civilians and an Iraqi interpreter. A U.S. soldier and two civilians were wounded.
Also Thursday, a rocket-propelled grenade struck a U.S. military vehicle in northern Baghdad, killing a soldier and wounding three others.
On Wednesday, a roadside bomb killed two troops southwest of Baghdad.
In addition, two Marines were killed in combat operations in Iraq's Anbar province on Wednesday.
With the deaths, 3,538 U.S. military personnel have died in the Iraq war -- 68 of them in June. Seven civilian contractors working for the military also have been killed since the 2003 invasion.
Gates: Troops anticipated the high level of combat
Defense Secretary Robert Gates said Thursday the spike in violence was because "our troops and the Iraqi troops are going into areas where they haven't been for some time."
"They anticipated that there would be a high level of combat," he said.
Simultaneous operations are being carried out as a part of Operation Phantom Thunder, a "powerful crackdown to defeat extremists in Iraq," according to a U.S. military statement released Wednesday.
U.S. and Iraqi military forces are focusing on insurgents in "Baghdad belts," a U.S. military commander told CNN on Wednesday.
Lt. Gen. Ray Odierno, the commanding general of multinational forces in Iraq, said it is part of a large-scale effort unfolding "in several different areas within Iraq."
Coalition forces launched Phantom Thunder on Saturday, the U.S. military said.
A push focused on the area around Baquba and Diyala province alone involves about 10,000 troops, the military said.
Joint Chiefs of Staff Chairman Gen. Peter Pace said the success of the new offensive should not be measured by the violence.
"If you try to define this in terms of level of violence, you've really put yourself on the wrong metric," he said.
"The metric really should be for Iraqi citizens, do they feel better about their lives today than they did yesterday? And do they think they're going to feel better about their lives tomorrow than they do today?"
Progress must be made, Pace said, so the Iraqi people can have confidence in their government.
Congress has called on Iraqi leaders to meet 18 political benchmarks to help measure the progress by September.
Some of those benchmarks include reforming constitutional provisions that allowed for the creation of autonomous federal regions; allowing former members of ousted dictator Saddam Hussein's Baath Party to return to public jobs; and passing laws that govern the distribution of the country's oil revenues.
Kurds and Shiites live in oil-rich regions, unlike the Sunnis, who want a fair share of the country's energy proceeds.
"What we're trying to do is to get -- for the Iraqi government -- enough space inside of which that they can do the good governance that they promised that they will do with regard to the laws that they're going to pass and the economics," Pace said.
CNN's Mohammed Tawfeeq and Saad Abedine contributed to this report.
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