The deal marks a new chapter in the US military's role since the 2003 invasion that toppled Saddam Hussein as it could once again see them involved in full-scale operations in urban areas.

General Ray Odierno, the top US officer in Iraq, told reporters training had began three weeks ago in the provinces of Kirkuk, Nineveh and Diyala and that about 70 percent of the tripartite force's checkpoints had been established.

"By January 31 they (checkpoints) will all be manned and we will begin joint security patrols," he said, without specifying how many American soldiers had been deployed.

"It is about protecting the population... who have been targets of Al-Qaeda and others who are trying to exploit political differences.

"This force has been put together in an attempt to stop the vulnerability of people in the disputed areas."

Kurdish leaders want their autonomous region, which currently consists of three distinct provinces, to be expanded into historically Kurdish-inhabited parts of Nineveh and Diyala as well as all of Kirkuk.

Baghdad, however, says the Kurdish region's borders should not extend past its existing provinces of Arbil, Sulaimaniyah and Dohuk.

The US military, which presently has 107,000 troops in Iraq, believes that tension between Sunni Arabs and Kurds is the main driver of instability in the country and a major threat to its long term future.

The deal to set up the joint force was first mooted by Odierno in August last year and he held discussions with Iraqi Prime Minister Nuri al-Maliki and Kurdish regional president Massud Barzani, whose approval was needed.

US combat troops exited Iraq's cities, towns and villages in June last year and are scheduled to leave completely by August. All American soldiers must withdraw from the country by the end of 2011 under a landmark security agreement signed between Baghdad and Washington.

Odierno did not reveal the conditions for establishing the new tripartite force but has previously said the need for US soldiers to be stationed in villages in the disputed zones could require an exception to be made to the existing security accord.

"There'll be some political challenges to it but we will work our way through it," Odierno said. "I have spoken to most of the local political leaders and Deputy Prime Minister (Rafa) Essawi (a Sunni) has helped me with that."

Odierno said that some Kurdish Peshmerga fighters had been killed in the disputed zones but there had yet been no attacks on the new, joint force.

"I have been very pleased with how it has gone so far. Cooperation has been very, very good," he added.