US forces are carrying out operations against Shiite insurgents, almost a year after the military announced a formal end to its combat operations in Iraq, US Defence Secretary Leon Panetta said on Monday.

"We have to unilaterally be able to go after those threats. We're doing that," Panetta told reporters, when asked about Iran-backed Shiite militias which Washington blames for attacks on US troops.

Washington has stepped up accusations that Iran is supplying more lethal weapons to Shiite militants in Iraq.

Panetta said Iraqi security forces must be pushed to take action against those groups. He said 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 force formally ended combat operations at the end of last August.

About 46,000 US troops remain in Iraq, down from a high of 170,000 after the 2003 US-led invasion. They are scheduled to leave in less than six months unless a deal is reached between Baghdad and Washington.

Panetta's arrival in Baghdad on Sunday coincided with a US soldier's killing in southern Iraq, the military's third death this month. June was the worst in three years for US forces, with 14 soldiers killed, most in rocket attacks.

Three rockets also slammed into Baghdad's heavily-fortified and sprawling Green Zone, which houses the US embassy, together with 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.