The new secretary of defense, Ashton Carter, said Saturday the U.S. may slow down the withdrawal of American troops from Afghanistan. Carter made the announcement during his first trip to Afghanistan as defense secretary.

On the flight there, Carter also discussed an impending battle against the Islamic State of Iraq and Syria, or ISIS, in Iraq - the battle for Mosul.

The Mosul offensive is expected to be the U.S.-led campaign's most difficult and strategically important battle yet.

The Iraqi city is the largest held by ISIS, with a population of one million, occupied by an estimated 1,000 to 2,000 militant fighters.

A big question is how involved U.S. military advisers will get, like whether they will accompany Iraqi troops on the ground to call in airstrikes.

On his way to Afghanistan on Friday, Carter left open that possibility.

"I'm always open to advice from our military commanders about what the best way to achieve success is," Carter said.

But unlike another Pentagon official, who earlier this week told reporters that U.S. and Iraqi forces will try to retake Mosul in April or May, Carter refused to say when it would be.

"I think the important thing is that it'll get done when it can be done successfully. Even if I knew exactly when that was going to be, I wouldn't tell you," Carter said.

A senior Obama administration official told CBS News that neither Carter nor the White House knew about that briefing, where a military official also said the assault would include between 20,000 and 25,000 Iraqi troops backed by U.S. air power.

While the U.S. and Iraqis have been telegraphing a Mosul assault at some point, detailing war plans is rare. The disclosure drew strong criticism from Republicans, including Sens. John McCain and Lindsey Graham.

The two Armed Services Committee members sent a letter to the president, demanding to know if the White House had approved the briefing, writing, "Those responsible have jeopardized our national security interests and must be held accountable."

U.S. officials expect that retaking Mosul will be a tough, potentially bloody battle. This seems to be a form of psychological warfare, possibly trying to convince ISIS fighters to leave Mosul before any assault or, on the other hand, forcing ISIS to bring in fighters to distract from other battlefronts.