American forces in Iraq are going after Iran-backed Shiite insurgents, who have killed 17 US soldiers in five weeks, US Defence Secretary Leon Panetta said in Baghdad on Monday.
"We have to unilaterally be able to go after those threats. We're doing that," Panetta told reporters, nearly a year after the US military declared an end to combat missions in Iraq.
"We lost a heck of a lot of Americans as a result. We can't allow this to continue," he said in a meeting with about 150 US troops at the Camp Victory base near Baghdad airport.
General Lloyd Austin, commander of US military forces in Iraq, told reporters that unilateral action "could include a lot of things."
"If there's no way to do that with the Iraqi security forces, then I'll patrol around my perimeter and do what needs to be done to ensure that my troops are protected," he said.
Panetta, who took over on July 1 from Robert Gates, said he would take all steps needed for the safety of the 46,000 US troops remaining in Iraq, down from a high of 170,000 since the 2003 US-led invasion that toppled Saddam Hussein.
Three US soldiers have been killed in Iraq so far this month, the last on Sunday when Panetta arrived in Baghdad from a visit to Afghanistan. June was the deadliest month for US forces in Iraq since 2008, with 14 soldiers killed.
The resumption of attacks against US troops comes as Iraqi leaders near decision time on whether they want to maintain a contingent of soldiers after 2011 when all US troops are scheduled to pull out.
Panetta said Iraqi security forces must be pushed to take action against the Shiite groups. US forces had carried out both joint operations with the Iraqis as well as unilateral missions against militants.
"The effort here obviously has to be to push the Iraqis to take on responsibility of going after some of these Shiite groups, going after those who use those kind of weapons," Panetta said.
He called for "pressure on Iran not to engage in this kind of behaviour."
Tehran itself has denied US accusations of smuggling weapons to insurgents in Iraq and Afghanistan.
Colin Kahl, a Panetta advisor, told reporters that US forces retained the right to carry out combat operations in Iraq. "We have self-defence authorities under the security agreement (with Iraq) to take on our own measures," he said.
US forces formally declared an end to combat missions last August.
Also on Monday, three rockets slammed into Baghdad's heavily-fortified and sprawling Green Zone, which houses the US embassy, other foreign missions and Iraqi government offices.
A woman and her three children were wounded in the attack, Iraqi security officials said, adding the rockets were launched from the Shiite neighbourhood of Zafraniyah in southern Baghdad.
Panetta was due to meet President Jalal Talabani and Prime Minister Nuri al-Maliki inside the Green Zone later on Monday. Rocket attacks against the Green Zone, which houses several embassies and Iraqi ministries, are common but casualties are rare.
Panetta is the latest top US official to arrive in Iraq asking officials to accept a contingent of American troops beyond 2011. US diplomatic sources in Baghdad say there has been no talk on the possible number who could remain.
A possible extension would be deeply unpopular among the public in Iraq, where many people look upon the American soldiers as "occupiers."
Some Kurdish officials have said they want US forces to stay beyond the deadline, but the powerful Shiite movement of radical cleric Moqtada al-Sadr has threatened to resume armed attacks on American troops if they extend their stay.
Panetta said he would also press Iraqi leaders to speedily appoint defence and interior ministers, posts which have stayed vacant due to political bickering since the formation of a unity government last December.