Barack Obama has give an ominous warning of bloodshed and violence in Iraq as US troops began their phased withdrawal.

He hailed as an "important milestone" the transfer of power to Iraqi security forces of all cities and towns. But he said the country was far from settled, citing the "senseless" car bombing in the city of Kirkuk yesterday that killed 27 people.

"Iraq's leaders must now make some hard choices necessary to resolve key political questions to advance opportunity and provide security for their towns and their cities," the US President said at the White House. "Make no mistake, there will be difficult days ahead.

"We know that the violence in Iraq will continue... there are those who will test Iraq's security forces and the resolve of the Iraqi people through more sectarian bombings and the murder of innocent civilians.

"I am confident that those forces will fail. Today's transition is further proof that those who have tried to pull Iraq into the abyss of disunion and civil war are on the wrong side of history."

More than 4,000 US servicemen have been killed in Iraq since the 2003 invasion. Mr Obama had vowed, as an election pledge, to bring US troops home at the earliest opportunity.

White House spokesman Robert Gibbs was asked if the withdrawal would see Mr Obama claim victory.

"We'll keep the banner printers from doing anything crazy," Mr Gibbs said, in an apparent reference to a notorious "Mission Accomplished" poster displayed on a US aircraft carrier visited by President Bush in May 2003.

Mr Obama said: "The fact that Iraqis are celebrating this day is a testament to the courage and capability and commitment of every single American who has served in Iraq."

Iraqi security forces jubilantly paraded in tanks and armoured vehicles as they took control of towns and cities, under an agreement between the US and Iraqi governments last year, but the celebrations were marred by the Kirkuk car bomb.

A poll by CNN published today reveals that three quarters of Americans are in favour of US troops pulling out.

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