Several mortars or rockets were fired into the fortified Green Zone of the Iraqi capital Baghdad, shortly after the arrival of US vice-president Joe Biden.
Iraqi police say two out of four suspected mortar rounds landed near the sprawling U.S. embassy compound but did not hit it.
Mr Biden was not hurt. He had arrived earlier on a visit aimed at reducing tensions between Kurdish, Shia and Sunni Arab communities.
The attack came after the senior American commander in Iraq had told the BBC that he was concerned that Iraq will be forgotten amid the current focus on Afghanistan.
General Ray Odierno suggested there was always a risk that the security situation in Iraq could deteriorate and said he wanted to finish what had been started.
The US has 130,000 troops in Iraq, almost twice the number in Afghanistan, but is due to drop to about 50,000 by this time next year.