The new US ambassador to Iraq, Ryan Crocker, has been sworn into office.
Mr Crocker was previously posted in Islamabad
Mr Crocker, an Arab speaker and former Pakistan ambassador, told embassy staff in Baghdad that Iraq was the US's "most critical foreign policy challenge".
The 57-year-old urged Iraqi Prime Minister Nouri Maliki to take "all the necessary steps" to unite the country.
Meanwhile, several police officers were arrested over the murder of more than 70 Sunni men in the town of Talafar on Wednesday. They were later freed.
The killings were in apparent reprisal for bombings in a Shia area on Tuesday, which left about 55 people dead.
Interior Minister Jawad Bolani told Iraqi television that police involvement in the murders would be investigated.
Mr Crocker broke with protocol by holding his swearing-in ceremony in Baghdad rather than in the US.
Addressing his embassy staff in Arabic, he told them: "You are the heroes of the country, in the true meaning of the word."
Zalmay Khalilzad has become the new UN envoy
He promised a "full unity of effort" with the US military and gave his backing to President George W Bush's handling of the US involvement in the country.
"President Bush's policy is the right one. There has been progress, there is also much more to be done," he said.
And he said Mr Maliki's government had to continue to deliver "tangible improvements" to the lives of ordinary Iraqis.
Mr Crocker replaces Zalmay Khalilzad, who was named in January as America's new envoy to the UN after 18 months in Baghdad.
The change of personnel comes as President Bush's policy of sending thousands more troops to Iraq has come under fire from political opponents in the US.
The Democratic-led Senate has voted in favour of setting a deadline of next March for withdrawing combat troops from Iraq.
Mr Bush has vowed to veto any bill setting out a timetable for withdrawal.