Secretary of Defense Leon Panetta makes an unannounced visit to Camp Dwyer in Iraq on Sunday.

BAGHDAD — The U.S. will not “walk away” from the challenge of Iran’s stepped-up arming of Iraqi insurgents who are targeting and killing American troops, U.S. Defense Secretary Leon Panetta said today.

“We’re very concerned about Iran and the weapons they’re providing to extremists in Iraq,” he told a small group of soldiers on his first visit to Iraq as Pentagon chief.

“We cannot sit back and simply allow this to continue to happen” he said. “This is not something we’re going to walk away from. It’s something we’re going to take on head on.”

Panetta said Iraq must more aggressively go after the Shiite militias that are using what he called Iranian-supplied weapons.

Three rockets fired from a mainly Shiite neighborhood hit Baghdad’s Green Zone during Panetta’s visit, Iraqi police said. No casualties were reported.

Panetta was visiting the U.S. military’s Camp Victory on the capital’s western outskirts at the time of the attack on the Green Zone, the heavily secured district in central Baghdad that is home to the U.S. and other embassies as well as Iraqi government offices.

Panetta also will huddle with the top U.S. military and diplomatic representatives in Baghdad before meeting with Iraqi leaders to discuss the possibility of keeping some U.S. troops in Iraq beyond 2011. He also will press Iraq for stronger action to stop stepped-up attacks on U.S. forces.

Panetta is meeting separately with Army Gen. Lloyd Austin at his headquarters outside Baghdad and with Ambassador James Jeffrey.

Later, he talks to Prime Minister Nouri al-Maliki and President Jalal Talabani.

The Obama administration believes Iraq needs a slimmed-down U.S. military presence beyond 2011, when virtually all U.S. troops are scheduled to depart. Many Iraqi leaders agree, but they’ve been unwilling to make a formal request.

There are now 46,000 U.S. troops in Iraq.