American forces are pursuing Iran-backed insurgents in Iraq, Defence Secretary Leon Panetta said in Baghdad on Monday as US deaths spike nearly a year after US troops formally ended combat operations.
"We have to unilaterally be able to go after those threats. We're doing that," Panetta said.
"We are very concerned about Iran and weapons they're providing to extremists here in Iraq. We lost a heck of a lot of Americans as a result. We can't allow this to continue," he told troops at US Camp Victory near Baghdad airport.
General Lloyd Austin, commander of US military forces in Iraq, told reporters 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 still in Iraq, down from a high of 170,000 since the 2003 US-led invasion that toppled Saddam Hussein.
"My first responsibility as secretary of defence is to make sure that I do everything necessary to protect you," Panetta later told NBC News.
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 approach 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."
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Tehran has denied US accusations of smuggling weapons to insurgents in Iraq and Afghanistan.
"The United States is not in a good position in Iraq and Afghanistan. They are doing everything they can to maintain their military presence in these two countries," Iran's state television website quoted foreign ministry spokesman Ramin Mehmanparast as saying.
In meetings with Iraqi President Jalal Talabani and Prime Minister Nuri al-Maliki later on Monday, Panetta encouraged them to be "more aggressive" against all extremist groups, his spokesman said.
"He made clear that we're neither pressuring nor pleading for US troops to stay here and that time is running out for a decision," Doug Wilson told reporters.
He said Panetta also had stressed for Maliki to speedily appoint ministers of interior and defence -- which he has failed to do since forming a unity government in December, and amid infighting among political groups.
"The secretary feels that he made progress on a clear understanding of the situation from them," Wilson said.
Meanwhile, Austin said insurgents were using more lethal weapons, and using them more effectively.
"Their targeting process is maturing," he said.
"They're working harder and harder to try to perfect their ability to target," Austin added, saying they were getting better training, presumably from Iran.
Major General Jeffrey Buchanan, spokesman for US forces in Iraq, last month identified the Iran-backed groups as Ketaeb Hezbollah, Asaib Ahel al-Haq and the Promised Day Brigade.
The latter is directly linked to Moqtada al-Sadr, a radical, anti-American Shiite cleric who divides his time between Iran and the city of Najaf in southern Iraq.
The other two groups are offshoots of Sadr's now-disbanded Mahdi Army, which fought against Iraqi and US-led coalition forces between 2004 and 2007, and which has been identified by the Pentagon as the main threat to stability.
Colin Kahl, a Panetta adviser, told reporters 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.