Baghdad (CNN) -- U.S. Defense Secretary Leon Panetta reiterated Monday that Iraq needs to crack down on armed factions that have been targeting U.S. troops with Iranian-supplied weapons.
"We are very concerned about Iran and weapons they are providing to extremists here in Iraq, and the reality is that we've seen the results of that," Panetta told troops in Baghdad. "In June, we lost a hell of a lot of Americans."
The remaining U.S. force of 46,000 has come under increased attacks in recent weeks, with 14 killed by hostile fire in June and three more in the first 10 days of July. The latest was reported shortly after Panetta arrived in Baghdad from Afghanistan, the scene of the other U.S. war in the region.
"We cannot just simply stand back and allow that to continue to happen," Panetta said.
The defense secretary said the United States will push Iraq to go after Shiite militia groups and will do so unilaterally as well.
"I want to assure you that this is something we're not going to walk away from," he said.
The strikes have increased, as Iraqi leaders debate whether to request an extension of the U.S. presence beyond the end of 2011 when the U.S. contingent is scheduled to withdraw.
Adm. Mike Mullen, the chairman of the Joint Chiefs of Staff, said last week that Iran has stepped up supplies of weapons to anti-American Shiite groups, possibly in order to claim credit for the scheduled pullout.
On Sunday, radical anti-American Shiite cleric Muqtada al-Sadr -- whose followers once fought pitched battles with U.S. and Iraqi government troops -- said he will activate his Promised Day Brigade to attack U.S. troops.
The U.S. military on Monday contended that the Promised Day Brigade is already operational and has claimed responsibility for several attacks.
"So this is much more than a threat -- they are already openly responsible for contributing to the ongoing violence in the country," said Col. Barry Johnson.
A few hours after Panetta spoke, three rockets landed inside the heavily fortified Green Zone in central Baghdad, officials with Iraq's Interior Ministry told CNN.
There were no casualties.
Officially called the International Zone, the Green Zone houses Iraq's government offices and foreign embassies.
On Sunday, three other rockets also landed near the area.
The United States invaded Iraq in 2003, arguing that then-Iraqi strongman Saddam Hussein was secretly harboring forbidden stockpiles of weapons of mass destruction that he could have provided to terrorists. Iraq was later found to have dismantled those weapons programs under U.N. supervision in the 1990s, though it had tried to conceal some weapons-related research from inspectors.
A bloody insurgency against U.S. and coalition forces followed the invasion, with the country erupting into a spasm of sectarian warfare between its Sunni minority and Shiite majority in 2006. Nearly 4,500 Americans and 300-plus coalition troops died in Iraq, while the Iraqi toll has been estimated at more than 100,000.
Much of the violence subsided after a counterinsurgency campaign launched in 2007, which brought the U.S. force to a peak of about 170,000.