US President Barack Obama urged national unity Saturday as he expressed gratitude to troops who fought the war in Iraq and welcomed them home.

"Now it is up to us to serve these brave men and women as well as they serve us," Obama said in his weekly radio and Internet address.

"Every day, they meet their responsibilities to their families and their country," he continued. "Now it's time to meet ours — especially those of us who you sent to serve in Washington."

The president said political divisions and discord should not stand in the way of progress in America.

"This is a moment where we must come together to ensure that every American has the chance to work for a decent living, own their own home, send their kids to college, and secure a decent retirement," he said.

US forces formally marked the end of their mission in Iraq with a low-key ceremony near Baghdad on Thursday.

The "casing of the colours" near the airport, the first site the US occupied in Baghdad in 2003, came with around 4,000 US soldiers still in Iraq, all of whom will depart in the coming days.

After that, all that will remain are 157 soldiers under the authority of the American embassy in Iraq, a country where there were once nearly 170,000 troops in more than 500 bases.

The withdrawal ended a war that left hundreds of thousands of Iraqis and nearly 4,500 American soldiers dead, many more wounded, and 1.75 million Iraqis displaced, after the US-led invasion unleashed brutal sectarian fighting.

Obama called the end of the war in Iraq "an extraordinary achievement" that was made possible by the hard work and sacrifice of US troops, arguing that it was time for the nation to address the needs of these veterans.

"After years of rebuilding Iraq, it is time to enlist our veterans and all our people in the work of rebuilding America," he said.