US Secretary of State Condoleezza Rice flew into Baghdad today for an unannounced visit, Iraqi officials said.
She was likely to hold meetings with Iraqi government officials, including Prime Minister Nuri al-Maliki.
Ms Rice has been on a tour of the Middle East, having already visited Israel, the Palestinian territories, Egypt and Saudi Arabia.
US officials have been putting pressure on Mr Maliki's Shia-led government to rein in militias blamed for much of the sectarian violence ravaging the country.
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Washington has made the training of Iraqi security forces the focus of hopes to start withdrawing 140,000 US troops.