Washington (AFP) - US military advisors are likely to take a more direct role in the ground campaign against jihadists in Iraq once Iraqi forces are ready to go on the offensive, the top US officer said in comments aired Sunday.

General Martin Dempsey, the chairman of the Joint Chiefs of Staff, said he had not yet encountered a situation where US air strikes would be more effective if US troops were on the ground spotting targets.

But he said that when Iraqi forces are ready to take the offensive against the Islamic State jihadists who have overrun swaths of territory in northern and western Iraq, as well as in Syria, that would likely change.

"Mosul will likely be the decisive battle in the ground campaign at some point in the future," Dempsey said in an interview with ABC's "This Week," referring to the northern Iraqi city seized by IS militants in June.

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

Dempsey caused a stir last month when he told lawmakers that if he believed US military advisors should accompany Iraqi troops on the ground against IS fighters, he would recommend that to President Barack Obama.

Obama has insisted there would be no US boots on the ground.

National Security Advisor Susan Rice said on a separate Sunday talk show that US commanders have not asked to put US combat forces into Iraq and she did not anticipate such a request.

"We are not going to be in a ground war again in Iraq," Rice said on NBC's "Meet the Press."

"It's not what is required by the circumstances that we face and even if one were to take that step, which the president has made clear we are not going to do, it wouldn't be sustainable."

Former defense secretary Leon Panetta, however, said Obama "has to be open to whatever recommendations are made in order to ensure that we are effective in going after ISIS."

Speaking on CBS's "Face the Nation," Panetta said boots on the ground were needed to stop the Islamic State group, which is also known as ISIL or ISIS.

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"It doesn't have to be American boots on the ground, but you have got to have people on the ground who can identify targets and who can help us develop the kind of effective air strikes that are going to be needed if we're going to be able to undermine, destroy this vicious enemy that we're dealing with."

The Iraqi military's weak performance, meanwhile, is another lure for more direct US military involvement.

Dempsey revealed that US Apache attack helicopters had to be called in recently to repel an IS attack on Iraqi forces 20 to 25 kilometers (12 to 15 miles) from the Baghdad airport.

"Had 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," he said.