Iraqi Prime Minister Nuri al-Maliki told Iraqis on Tuesday that their own soldiers and police are up to the job as US forces ended a combat role after seven years of fighting that has cost thousands of lives.
A major troop pullout over past months has left less than 50,000 US soldiers in Iraq while a simultaneous surge in car bombings and shootings, many targeting local security forces, has raised security concerns.
US President Barack Obama was to mark the symbolic end of combat operations in a speech from the Oval Office at 0000 GMT (3 am on Wednesday for Baghdadis), after visiting a base in Texas where he was to meet returned Iraq veterans.
He was also expected to speak by telephone with former president George W. Bush who, backed by key ally Britain, took the decision to invade Iraq in March 2003, ousting dictator Saddam Hussein within weeks.
In advance of Obama's speech, Maliki said on state television that Iraq was a "sovereign and independent" state and he was confident the last US forces would leave the country as planned at the end of 2011.
"I reassure you that the Iraqi security forces are capable of taking full responsibility," Maliki said.
"Unfortunately we are facing a campaign of doubt."
US Vice President Joe Biden landed in Baghdad on Monday night to mark the American military's change of mission from combat to training and advisory tasks in support of Iraqi forces, starting from Wednesday.
More than 4,400 US troops have died in Iraq since the invasion, a number dwarfed by the estimated 100,000 civilians who have been killed, according to Iraq Body Count.
Obama declared shortly after taking office last year that the US combat mission in Iraq would end on August 31, 2010, after which American troops would take on a training and advisory role prior to a complete withdrawal in 2011.
There are now 49,700 American soldiers in Iraq, less than a third of the peak figure of almost 170,000 during the US "surge" of 2007, when the country was in the throes of Shiite-Sunni carnage that cost tens of thousands of lives.
The outgoing commander of US forces in Iraq, General Ray Odierno, has said the new force strength will be maintained "through next summer" before troop numbers fall towards zero by the end of the December 2011 withdrawal deadline.
He told the New York Times on Monday that failure to form a new government could undermine Iraqis' faith in democratic rule. "The longer that takes, the more frustrated they might get with the process itself," Odierno said.