"The forces are working in three provinces, Diyala, Kirkuk and Nineveh," Jabbar Yawer, spokesman of the autonomous Kurdish region's ministry of peshmerga (former rebel) fighters, told AFP.

Fifteen checkpoints, each with 15 peshmerga, have been established in Nineveh, which has a Sunni Arab majority and Kurdish minority, and contains seven of the country's 12 disputed zones.

There are the same number of US and Iraqi troops at each checkpoint, Yawer said.

Iraq's disputed zones include the northern oil province of Kirkuk, where 110 peshmergas have been deployed alongside the same number of US and Iraqi soldiers, said Yawer.

A further five checkpoints, again with 15 peshmerga, US soldiers and Iraqi troops, have been set up in Diyala province, which contains two disputed zones.

"The joint operations between the peshmerga, Iraqi and US army are in order to control the security situation in these areas, especially at the entrances and checkpoints of the disputed areas," Yawer added.

"These forces will prevent the terrorists from going into these areas."

General Ray Odierno, the top US army officer in Iraq, announced last month that American troops were working with Iraqi forces, both Arab and Kurdish, in the disputed areas, aiming to boost trust and reduce tensions between Sunni Arabs and Kurds. He did not, however, say how many US troops were involved.

Odierno first raised the prospect of joint operations in August last year, arguing that insurgents were taking advantage of poor cooperation between the mostly Arab Iraqi army and Kurdish security forces to launch attacks.

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.

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

Iraqi soldiers and Kurdish peshmerga have clashed several times in the past two years as Prime Minister Nuri al-Maliki's troops have tried to bolster their presence in and around the disputed areas.

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