BAGHDAD, Iraq (CNN) -- Six U.S. soldiers were killed Wednesday in Iraq when a bomb exploded in a booby-trapped house while they were on patrol north of Baghdad, the military announced.





U.S. soldiers conduct a house-to-house assessment mission in Baghdad on Wednesday.





Four soldiers were also wounded in the attack and evacuated to a coalition forces hospital, Multi-National Corps-Iraq said.





The troops were taking part in Operation Iron Harvest -- a new drive against Islamic militants in northern Iraq after a spate of attacks on local anti-insurgent groups.





The operation is part of a nationwide push against jihadists loyal to al Qaeda in the provinces of Diyala, Salaheddin, Nineveh, and Tameem.





"It will be a difficult fight, but we will continue to execute simultaneous operations in each one of our four provinces," said Maj. Gen. Michael Hertling, commander of U.S. troops in northern Iraq.





But resistance in the "breadbasket" region in eastern Diyala so far is "less than anticipated," he said.





The offensive -- which has been centered near Muqdadiya -- has left 20 to 30 suspected insurgents dead in the area around that city, Hertling said. About 24,000 U.S. troops, 50,000 Iraqi soldiers and 80,000 local police are based in the region.





In addition, about 15,000 Iraqis are taking part in anti-insurgent groups known as Concerned Local Citizens or Awakening Councils. The groups will perform defensive security tasks during the operation, the U.S. military said.





The new push comes just short of a year since President Bush ordered almost 30,000 additional troops to Iraq to secure Baghdad and its surrounding provinces.





Al Qaeda in Iraq has been deeply rooted in Muqdadiya, about 62 miles north of Baghdad, since about 2004.





Wednesday's deaths occurred in an agricultural area on the north side of the Diyala River from the city.





The U.S. military has conducted operations there in the past and has frequently come under fire. One recent operation ended with several insurgents killed and a large amount of weapons and explosives seized, a military source told CNN.





The bombing Wednesday was the first incident involving multiple deaths of U.S. soldiers in Iraq since September 10, when seven Multi-National Division-Baghdad soldiers died and 11 were injured in a vehicle accident in western Baghdad.





The last attack in which so many U.S. troops died from hostile action happened May 28, when six Task Force Lightning soldiers were killed by explosions near their vehicles during operations in Diyala province.





Awakening Councils





Iraq's concerned-citizens groups began forming in the country's predominantly Sunni Arab regions in late 2006, and their cooperation with U.S. forces against the jihadists has been credited with much of the decline in violence in Iraq since summer.





But that cooperation has made them an increasing target for al Qaeda in Iraq, which Hertling said has launched a campaign of intimidation against the locals.





Monday, five severed heads were left on a road leading to Baquba "with Arabic writing in blood on the forehead which said, 'Join the Concerned Citizens and you will end up like this,' " Hertling said.





And he played video from an aerial surveillance drone that showed three people in Diyala assassinating another person -- pulling the victim from a vehicle, shooting him and leaving him in a ditch. They were eventually captured, and information was found linking them to al Qaeda in Iraq, he said.





Hertling said that even though there has been a reduction in attacks across the country, there has been an increase in "high-profile, spectacular" events in his region, citing a suicide vest attack and a bridge bombing that sparked media attention and made the region look as if it were "reeling."





But he said attacks like those will prove to be the jihadists' "Achilles' heel," turning the population against them and driving them toward the concerned citizens' groups.





"It's sort of a reverse counterintuitive logic," Hertling said. "They are trying to intimidate people that join them by killing them, and it's causing more people to go against them."
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