WASHINGTON (Reuters) - U.S. troops will probably need to play a bigger role “advising and assisting” Iraqi forces on the ground in the future, the highest ranking U.S. military officer said in an interview broadcast on Sunday.

Chairman of the Joint Chiefs of Staff Gen. Martin Dempsey speaks in a press briefing at the Pentagon in Washington, September 26, 2014. REUTERS/Larry Downing

U.S. forces have been bombing Islamic State fighters in Iraq since August and expanded the air campaign to Syria last month. The White House has repeatedly said it will not send ground troops back to Iraq, where President Barack Obama withdrew U.S. forces in 2011.

But General Martin Dempsey, the chairman of the joint chiefs of staff, said the role of U.S. forces could be expanded in future if Iraqi troops try to recapture Mosul, the main city in northern Iraq, which Islamic State fighters seized in June.

“Mosul will likely be the decisive battle in the ground campaign at some point in the future,” Dempsey said in an interview aired on ABC’s “This Week”.

“My instinct at this point is that that will require a different kind of advising and assisting, because of the complexity of that fight,” he said.

The United States used army Apache attack helicopters for the first time this past week to provide close air support to Iraqi forces. Use of the low-flying helicopters is far riskier than bombing from jets but allows closer cooperation with troops engaged in combat on the ground.

Dempsey said the decision to use the Apaches was taken to halt fighters who might otherwise have been able to attack Baghdad’s airport.

“They overrun the Iraqi unit it was a straight shot to the airport. So, we’re not going to allow that to happen. We need that airport,” said Dempsey.

The airport lies on the western outskirts of Baghdad near territory where fighters have been active for months.

U.S. air strikes have so far helped Kurds and government troops take back some territory in the north of Iraq but have not halted the fighters on the capital’s Western outskirts or prevented them from striking inside the city.

At least 45 people were killed in car bomb attacks in Baghdad on Saturday. On Sunday, fighters killed 28 people in an attack on a Kurdish security headquarters in the north and assassinated the police chief in the province west of Baghdad.

Last month, Dempsey raised the possibility that he might have to advise sending American ground troops into Iraq in future, although the White House says it will not do so.

National Security Adviser Susan Rice told NBC’s Meet the Press on Sunday that ground combat by U.S. troops was ruled out.

“We’ll do our part from the air and in many other respects in terms of building up the capacity of the Iraqis and the Syrian opposition, the moderates. But we are not going to be in a ground war again in Iraq,” she said.

Senator John McCain, ranking member of the Senate Committee on Armed Services, said on CNN on Sunday that Obama’s strategy against the Islamic State was failing. He said he did not think Islamic State fighters could take Baghdad but that they could take the airport and infiltrate Baghdad with suicide bombers.

“They’re winning and we’re not,” McCain said. “Pinprick bombing is not working.”

Dempsey, will convene a meeting of more than 20 foreign defense chiefs next week outside Washington to discuss the multinational campaign against Islamic State.