Khalilzaid told reporters the region is interconnected and that "what happens in Lebanon affects things here."
The United States has repeatedly accused Iran of fomenting violence and instability by sending weapons and fighters into Iraq, a charge Tehran denies.
Khalilzad was speaking at a ceremony marking the U.S. military handover of security responsibilities to Iraqi forces in the northern city of Tikrit.
He said efforts are under way to end the sectarian violence in Iraq, including meetings between rival militias. He said there is an effort to reach what he called a "moral compact" between religious forces to stop the violence.
Iranian Shi'a protesting the Golden Mosque Bombing in Iraq on February 24
WHAT IS GOING ON? On March 8, RFE/RL's Washington office hosted a roundtable discussion on relations between Iraq and Iran. Although most analysts agree that Iran has been actively involved in Iraq since the U.S.-led military operation to oust former Iraqi President Saddam Hussein, they continue to debate the nature, extent, and intent of that involvement.
The RFE/RL briefing featured WAYNE WHITE, former deputy director of the U.S. State Department Bureau of Intelligence and Research's Office of Analysis for the Near East and South Asia, and A. WILLIAM SAMII, RFE/RL's regional analyst for Iran and editor of the "RFE/RL Iran Report."
LISTENListen to the complete RFE/RL briefing (about 75 minutes):
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