U.S. Defense Secretary Leon Panetta says U.S. forces in Iraq are taking unilateral action against Shi'ite militants who he says are using Iranian-supplied weapons to stage deadly attacks on American soldiers.

Fourteen U.S. soldiers were killed last month in attacks mostly blamed on Iraqi Shi'ite insurgents. It was the highest monthly combat-related death toll for U.S. forces in Iraq in three years. This month, attacks have killed three more U.S. soldiers, one of them on Sunday, when Panetta arrived in Baghdad on his first visit since taking the defense post this month.

In remarks at a U.S. military base near Baghdad Monday, Panetta said the United States will not “stand back” as attacks continue. He said U.S. forces instead are responding to such threats unilaterally.

The commander of U.S. forces in Iraq, General Lloyd Austin, said Iraqi militants are making more effective use of their Iranian-supplied weapons. He said U.S. troops are doing what is necessary to protect themselves.

U.S. forces ended combat operations in Iraq last August under a 2008 U.S.-Iraqi security agreement that largely relegates them to training and supporting Iraqi security personnel.

Panetta also met Monday with Iraqi Prime Minister Nouri al-Maliki in Baghdad's Green Zone, to urge his government to fight Shi'ite militias more aggressively. Before the talks, Panetta said he also wants Iraq to agree on a defense minister after a year-long political stalemate, and decide whether it wants U.S. forces to stay beyond an end-of-year deadline.

The 2008 security agreement requires U.S. forces to leave Iraq by December 31. U.S. and Iraqi officials have expressed concern about the ability of Iraqi government forces to cope with security after the pullout.

In his comments to U.S. soldiers, Panetta said he is frustrated by the delays in Iraq's decision-making process, but he recognizes that they reflect the “nature of democracy.”

The U.S. defense chief also said he is focused on defeating al-Qaida, which he says has about 1,000 fighters in Iraq. He told the soldiers that “the reason you guys are here is because on 9/11 the United States got attacked, and 3,000 human beings got killed because of al-Qaida.”

He later tried to clarify that remark, saying he did not mean to suggest the September 11, 2001 terrorist attacks were a justification for the 2003 U.S.-led invasion of Iraq. He said there was a need to fight al-Qaida because it developed a lethal presence in Iraq after the invasion.