WASHINGTON - President Barack Obama's military campaign against the Islamic State group already has extended beyond the limits he first outlined.

But military experts inside and outside the administration argue that an even greater expansion may be needed for the mission to succeed, including positioning U.S. ground troops with front-line Iraqi security forces.

Doing that could put Obama close to violating his pledge to keep Americans out of combat.

For Obama, re-engaging in combat in Iraq would mean going back on promises about the current mission and undercutting a pillar of his presidency - ending long wars and avoiding new ones.

If commanders request ground troops and he turns them down, Obama could be accused of putting his legacy first.

Obama has shown a willingness to expand the size and scope of the fight against the Islamic State extremist group.

He first announced a limited airstrike campaign, but the U.S. now is pursuing targets across Iraq and is expected to push the attacks into Syria.

About 1,600 U.S. troops have been sent to Iraq to train local security forces and protect U.S. personnel. Soon, the Pentagon will start training and equipping Syrian rebels to fight the militants.

Obama has said that Iraqi security forces, the Kurdish peshmerga and the Syrian opposition fighters will wage ground battles with the Islamic State group. But some of Obama's current and former military advisers have said that unless American troops also participate, it will be difficult to defeat the militants.

"They're not going to be able to be successful against ISIS strictly from the air, or strictly depending on the Iraqi forces or the peshmerga," said Robert Gates, Obama's former defense secretary, using one of the acronyms for the Islamic State group. "So there will be boots on the ground if there's to be any hope of success in the strategy."

Gen. Martin Dempsey, chairman of the Joint Chiefs of Staff, told Congress he would recommend that step if Obama's initial strategy fails. Dempsey also said that about half the Iraqi army is incapable of partnering effectively with the U.S. to combat the Islamic State group, suggesting a high likelihood that more Americans would be needed on the ground.

The president responded swiftly to Dempsey's comments by emphasizing his pledge to keep Americans out of combat missions.

"The American forces that have been deployed to Iraq do not and will not have a combat mission," Obama told troops at U.S. Central Command in Tampa, Fla. "As your commander in chief, I will not commit you and the rest of our armed forces to fighting another ground war in Iraq."

But White House officials have left open the possibility that Obama could accept a recommendation to put ground troops in forward operating positions alongside Iraqi and peshmerga forces.

While officials say those troops would not be sent with a specific combat mission, they would be armed, as are the 1,600 military personnel sent to Iraq this summer, and would have the authority to fight back if attacked.

Richard Fontaine, president of the Center for New American Security, said that would leave Obama with "something of a rhetorical quandary."

"From a realistic and even legal standpoint, what's going to be happening in Iraq is going to look a lot like combat," said Fontaine, a former State Department official who has advised Sen. John McCain, R-Ariz., on foreign policy.

While Obama has broad public support for the airstrikes, it's unclear whether the public would approve of an expanded ground mission. A recent CNN/ORC poll found that while 76 percent of Americans back strikes in Iraq and 75 percent support them in Syria, just 38 percent favor sending U.S. ground troops to those countries.

Should Obama have to take that step, the White House's goal would be to ensure that the ground forces were not solely American. Administration officials are pressing other countries, particularly Iraq and Syria's Arab neighbors, to commit ground troops to help bolster local security forces.

Administration officials say they've received positive feedback from Arab nations, but there have yet to be any specific commitments about ground troops or other direct action.

Obama will make further appeals when he arrives in New York Tuesday for meetings with foreign leaders at the annual U.N. General Assembly gathering.