Iraqi Defense Minister Khalid al-Obeidi, center, speaks to his soldiers after a military operation in Tikrit on Dec. 8. The commander of U.S. forces battling the Islamic State says 1,500 allied forces are expected to join the fight. (Hadi Mizban/AP)

U.S. allies are expected to send as many as 1,500 soldiers to Iraq to complement a growing American military force that will train and advise Iraqi troops in their battle against the Islamic State, a senior U.S. commander said Monday.

Lt. Gen. James L. Terry, who heads a new U.S. joint task force overseeing military operations in Iraq and Syria, said the United States is building a military coalition of nearly 40 countries that parallels a larger diplomatic alliance the Obama administration has assembled to counter the militant group, which, though slowed by U.S. and allied airstrikes, is attempting to expand its domain across Iraq and Syria.

As part of that effort, Terry is working to finalize allied troop contributions following a Dec. 2-3 conference he hosted to coordinate allied military efforts.

“I’m comfortable with the [U.S.] boots on the ground right now,” Terry told reporters in Kuwait, where the joint task force is located. “What I’ve got to do now is balance the coalition contributions.”

Terry said he hopes the number of troops that partner countries send will roughly equal the maximum 1,500 that President Obama authorized to be sent to Iraq last month. There are 1,650 U.S. troops in Iraq, but under current plans, that force is likely to grow to about 3,000.

Lt. Gen. James Terry heads a new U.S. joint task force overseeing military operations in Iraq and Syria. (AP)

“When you start now to balance the different capabilities out there across the coalition, I think we’re doing pretty well in terms of boots on the ground,” Terry said.

Nearly six months after the Islamic State burst out of Syria and seized much of northern Iraq, the Obama administration is seeking to expand military and financial support from allied nations that also see a threat from the group, which has promised to establish a fundamentalist caliphate across the Middle East and to strike the West.

The general declined to say which countries would provide troops in Iraq because commitments are still being worked out. But he said the coalition would include a “broad mix” of American allies, probably from Western nations and the Middle East.

“A lot of coalition countries come together, although with some different kinds of capabilities, with a common purpose out there to halt what ISIL is doing,” Terry said, using an acronym for the Islamic State.

Terry described the militant group as being on the defensive after more than 1,200 airstrikes since the summer. Although the Obama administration has suggested that the airstrikes — combined with efforts by Iraqi soldiers, Kurdish peshmerga forces and volunteer fighters on the ground — have contained the militants’ momentum, the Iraqi government has not been able to expel the group from the major population centers it controls.

Defeating the Islamic State is an administration priority not only because of the threat the militants pose to U.S. security but also because Iraq’s fate is so closely identified with the United States. In 2011, Obama pulled the last U.S. troops out of Iraq, declaring that the Iraqi government was able to stand on its own eight years after the U.S.-led invasion.

The new U.S. military action in the region has drawn unusual support from an array of Middle Eastern nations that fear the Islamic State’s violent fundamentalism, its appeal among some Sunni Muslims and its determination to upend the region’s existing order.

With the group’s advance slowed, the Obama administration is hoping to retrain a portion of the Iraqi army, which partially collapsed during the initial Islamic State assault in June.

Terry said the troops from allied nations, once they arrive in Iraq, will mostly take part in training and advising Iraqi forces.