US Secretary of State John Kerry has arrived in Iraq for an unannounced visit to press for greater co-operation over the conflict in Syria.
Mr Kerry will urge Iraqi leaders to stop Iranian flights over Iraq carrying arms and fighters for the Syrian government, US officials said.
The US has accused Iraq of turning a blind eye to the flights, which Iran claims transport only humanitarian aid.
Mr Kerry is also expected to discuss democratic reforms in Iraq.
Mr Kerry's visit comes days after the 10th anniversary of the US-led invasion of Iraq that toppled former Iraqi President Saddam Hussein.
It is his first trip to Iraq since taking office earlier this year, and the first by an American secretary of state since 2009.
One official told the Associated Press that the Iranian flights to Syria were crossing Iraq "close to daily".
Iraq said last year it would inspect the flights, but only two planes had been checked, the official said.
During his visit, Mr Kerry will also suggest that Iraq's government reconsider a recent decision to delay provincial elections in Anbar and Nineveh provinces, US officials said.
