The United States and coalition forces are 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, Defense Secretary Chuck Hagel told Congress on Thursday.

"ISIL's advance in parts of Iraq has stalled, and in some cases been reversed, by Iraqi, Kurdish, and tribal forces supported by U.S. and coalition airstrikes," Hagel said in testimony to the House Armed Services Committee. "But ISIL continues to represent a serious threat to American interests, our allies, and the Middle East ... and wields influence over a broad swath of territory in western and northern Iraq and eastern Syria."

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

The testimony of Hagel and Army Gen. Martin Dempsey, the chairman of the Joint Chiefs of Staff, comes just days after President Barack Obama asked Congress for a new $5.6 billion plan 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.

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."

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.

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Associated Press writer Lolita C. Baldor contributed to this report.