The chairman of the Joint Chiefs of Staff told Congress on Thursday that he wouldn’t rule out sending a small number of American forces to fight alongside Iraqi troops during some of the more complex missions against the Islamic State. 

"I'm not predicting at this point that I would recommend that those forces in Mosul and along the border would need to be accompanied by U.S. forces, but we're certainly considering it,” Army Gen. Martin Dempsey told the House Armed Services Committee. 

President Obama, since launching the mission, repeatedly has vowed that U.S. combat troops will not be fighting on the ground in Iraq. But Dempsey has appeared to leave some wiggle room. Dempsey's sober assessment echoed his testimony to Congress in September at the start of the campaign against the militants who have seized parts of Iraq and Syria.

Dempsey added Thursday that the U.S. has a modest force in Iraq now, and "any expansion of that, I think, would be equally modest. I just don't foresee a circumstance when it would be in our interest to take this fight on ourselves with a large military contingent." 

Joining him at the witness table was Defense Secretary Chuck Hagel, who said the coalition was making progress in the fight against Islamic State militants in Iraq and Syria, but the American people must prepare for a long and difficult struggle.

Hagel said the "pressure is having an effect on potential ISIL recruits and collaborators ... striking a blow to morale and recruitment. We know that. Our intelligence is very clear on that."

He used the term ISIL for the Islamic State of Iraq and the Levant, another term for the group. 

“We are at war against ISIL,” Dempsey said.

The testimony comes just days after Obama asked Congress for another $5.6 billion to expand the U.S. mission in Iraq and send up to 1,500 more American troops to the war-torn nation.

Obama authorized the deployment of advisory teams and trainers to bolster struggling Iraqi forces across the country, including into Iraq's western Anbar province where fighting with Islamic State militants has been fierce. Obama's plan could boost the total number of American troops in Iraq to 3,100. There are currently about 1,400 U.S. troops there, out of the 1,600 previously authorized.

Lawmakers expressed skepticism about limiting the U.S. deployment to advisers and trainers, with Rep. Howard "Buck" McKeon, R-Calif., the chairman of the Armed Services Committee, arguing that "limiting our advisers to headquarters buildings will not help newly trained Iraqi and Syrian opposition forces hold terrain, much less defeat ISIL in the field. Yet the president has doubled down on his policy of `no boots on the ground,' despite any advice you give him."

In citing expert advice, McKeon offered comments from previous defense secretaries and also quoted Duke basketball coach Mike Krzyzewski, who last month told an Army conference that ruling out ground forces is like telling a rival you won't play your best players.

Hagel maintained that the U.S. personnel will not be involved in ground combat.

Congress also must decide whether to reauthorize training and equipping of moderate Syrian rebels, an authority that expires Dec. 11.

Lawmakers are bracing for a broader fight next year over a new authorization to use military force to replace the post-Sept. 11 law and the one crafted for the Iraq war 11 years ago.

The Associated Press contributed to this report.