US Defence Secretary Donald Rumsfeld arrived in Iraq on Wednesday on a visit that comes amid waning support in the United States for the war and questions about when US troops might start returning home.
Rumsfeld, whose visit follows stops in Afghanistan and Tajikistan, has so far declined to comment on troop levels, saying those decisions should be made with ground commanders.
He will discuss security in Baghdad as well as the training and mix of Iraqi security forces.
The visit comes at a critical time, both for Prime Minister Nuri al-Maliki's US-backed national unity government and for the US military in Iraq, which faces a series of inquiries in which US troops are suspected of killing civilians.
The latest case, the suspected rape and murder of a teenage girl and her family by US soldiers last month, outraged Iraqis and prompted Maliki to call for a review of foreign troops immunity from Iraq's courts.
While Washington has resisted setting a timetable for withdrawal of its 127,000 troops, many of President George W Bush's Republican allies are anxious to show progress before US congressional elections in November.
But that could be complicated by a surge in sectarian bloodletting between Iraq's majority Shi'ites and minority Sunnis that has killed scores in street fighting and tit-for-tat attacks in Baghdad since Friday alone.
The violence has raised questions about the effectiveness of the new Iraqi army, which Washington is training to take over security so that it can begin pulling out its troops.
US ambassador Zalmay Khalilzad said in a speech on Tuesday that sectarian violence was now the biggest challenge facing US and Iraqi forces, overtaking the three-year-old Sunni insurgency as the main source of instability.