U.S. Secretary of Defense Leon Panetta says U.S. forces in Iraq will act against Iran's arming of Shi'ite militants blamed for a wave of deadly attacks on American soldiers.

Attacks by suspected Iraqi Shi'ite insurgents killed 14 U.S. soldiers in June. It was the deadliest month for U.S. forces in Iraq in two years. Two more U.S. soldiers have been killed in attacks in Iraq this month, one of them on Sunday.

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

The 46,000 U.S. troops in Iraq have the right to defend themselves under a 2008 U.S.-Iraqi security agreement. But, the deal gives Iraqi government forces the lead in combat operations and largely relegates U.S. forces to a training and support role.

Panetta was due to meet Iraqi leaders later Monday. He said he would urge Iraq to fight Shi'ite militias more aggressively and make a decision about whether they want U.S. forces to remain beyond an end-of-year withdrawal deadline.

The 2008 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.

Panetta also expressed frustration with the inability of Iraqi Prime Minister Nouri al-Maliki's coalition government to agree on a new defense minister after a year of political stalemate. But, the U.S. defense secretary also said the delay reflects the “nature of democracy.”