U.S. Army Chief of Staff Gen. Ray Odierno said U.S. may need more troops in Iraq and he would not rule out small numbers of troops in specialized combat missions. (Hans Pennink/AP)

Gen. Ray Odierno, the Army chief of staff, said Friday it might be necessary to deploy more U.S. forces to Iraq beyond the 1,600 troops already there, warning that the fight against the Islamic State will intensify and could go on for years.

Odierno, who served as the top U.S. military commander during the last war in Iraq, also said he would not rule out the need to send small numbers of U.S. ground troops into combat as tactical airstrike spotters or as front-line advisers embedded with Iraq forces.

In a breakfast interview with the Defense Writers Group, Odierno said that “1,600 is a good start” and that “I don’t think there’s a rush, a rush to have lots of people in there now.” But he predicated that as operations accelerate against jihadist fighters from the Islamic State, military commanders will revisit U.S. troop levels. “Based on that assessment, we’ll make further decisions,” he said.

President Obama has authorized the deployment of the 1,600 U.S. troops in several stages since June, most recently on Sept. 10, when he sent an additional 475 personnel to Iraq. Most serve as advisers to Iraqi and Kurdish forces or as security for the U.S. Embassy and the international airport in Baghdad.

While Obama has repeatedly insisted he will not send U.S. ground forces into combat in Iraq, he has not indicated whether he thinks more troops will be necessary in the coming months to carry out his strategy against the Islamic State.

Any recommendations from military commanders to send more troops to Iraq would have to receive the endorsement of Defense Secretary Chuck Hagel before going to the White House for final approval.

Asked if Hagel was open to the idea of deploying more troops, his spokesman, Rear Adm. John Kirby, said in an e-mail: “The Secretary is — and will remain — open to hearing the advice and counsel of senior military leaders. He expects that advice to be candid, forthright and forward-looking.”

Thousands of U.S. troops are stationed at bases in nearby Persian Gulf countries, from which they are carrying out a campaign of airstrikes and surveillance missions targeting the Islamic State, a group that has seized large parts of territory in Iraq and Syria.

On Friday, France joined the United States in the air war, conducting its first airstrike against Islamic State targets in Iraq. Rafale fighter jets destroyed an Islamic State supply depot near Mosul, according to President Francois Hollande. He said more French operations would follow in “coming days.”

Obama has sought to rally a broad international alliance to counter the extremist group. There have been pledges of support, yet to be detailed, but France is the first to accompany the United States in conducting airstrikes.

France has several Rafale fighters and other warplanes stationed at al-Dhafra Air Base in the United Arab Emirates. The U.S. military also maintains a force at al-Dhafra, a key staging area for its air operations over Iraq.

In his interview, Odierno said the fight against the Islamic State will become more difficult as Iraqi and Kurdish forces, with the help of U.S. air power and advisers, go on the offensive and try to retake territory.

“This is going to go on,” he said. “This is not a short term — I think the president said three years. I agree with that — three years, maybe longer. And so what we want to do is do this right. Assess it properly, see how it’s going, adjust as we go along, to make sure we can sustain this.”

Odierno said he supported the Obama administration’s current strategy to train and equip Iraqi, Kurdish and Syrian proxy forces to attack Islamic State insurgents, while keeping U.S. troops out of any firefights on the ground.

But he said targeting Islamic State fighters and leaders with U.S. airstrikes will become more complicated as they retreat from open spaces they once controlled and into major cities and other populated areas.

“The worst thing that can happen for us is if we start killing innocent Iraqis, innocent civilians,” he said. “So we have to be very careful and precise on how we’re doing this. We’ll have to determine that, as we go forward, if we can sustain the level of preciseness that is necessary to limit civilian casualties.”

Asked if it might become necessary to embed U.S. tactical air controllers or Special Operations Forces with Iraqi troops on the front lines, Odierno replied: “I don’t rule anything out. I don’t ever rule anything out, personally.”

Other U.S. commanders have also indicated that they would like Obama to give them more leeway and relent on his stated opposition to U.S. troops becoming involved in ground combat.

On Tuesday, Army Gen. Martin Dempsey, the chairman of the Joint Chiefs of Staff, testified before Congress that, if necessary, he might recommend to Obama in the future that he permit small teams to embed with Iraq forces.

Dempsey also acknowledged that Army Gen. Lloyd Austin, the commander for the Middle East, had already recommended doing so in the case of at least one battle in Iraq but was overruled.

There are signs that the White House is becoming more flexible. Obama aides said this week that the president might consider cases in which U.S. advisers could embed with Iraqi forces or call in airstrikes, although they shied away from describing such roles as combat missions.