US Secretary of State Condoleezza Rice arrived in Baghdad on an unannounced visit, Prime Minister Nuri al-Maliki's office said.
Rice arrived at a time when Washington and Baghdad are negotiating a long-awaited agreement that will allow US forces to stay in Iraq beyond the end of this year, when a UN Security Council mandate expires.
Iraqi officials say the deal is nearly finished, while the White House says it is not yet complete. Iraqi officials say they would like the deal to include "time horizons" for US forces to withdraw from the country.
Other issues that need to be sorted out include immunity for US troops from Iraqi law and the status of prisoners held by American forces.