Most of the troops will be stationed at the recently recaptured Qarayyah airfield, which is about 25 miles south of Mosul and will be a key staging area for the upcoming U.S. and Iraqi effort to retake that city from the terror group.
President Barack Obama approved the deployment. The new troop cap in Iraq is 4,647, according to the Pentagon, although there are currently just over 3,600 troops in the country.
A senior U.S. military official said the number of additional troops is what had been requested by Gen. Sean MacFarland, commander of the coalition against ISIS in Syria and Iraq, and the official added that there could be requests for more troops in the future.
Carter said the additional troops will deploy in days or weeks and had already received their orders.
"We need to be clear that there are Americans at risk here in Iraq, there have been, I've emphasized that right along," Carter told reporters in Baghdad. "They are doing all kinds of essential functions. Their basic strategic function is enabling but there is risk associated with that. Make no mistake about that."
MacFarland said U.S. forces "will not be any closer to the enemy than we've been anywhere else."
"But," he added, "we need to move to this place so we (will) be as close to the fighting as we have been in the Euphrates River Valley fights."
To make the retaken airfield fully functional, MacFarland said the U.S has to "bring in a wide array of capabilities."
"That's what these forces do," he added. "As always, when we bring forces to an area, we bring in a security envelope, there's a security element that goes with it."
The 560 new U.S. troops are specifically for the fight to recapture Mosul. They will assist the Iraqi troops on the Qarayyah base with logistics, because a large number of Iraqis will be sent there. But the Americans may also accompany Iraqi forces.
"At every step in this campaign, we have generated and seized additional opportunities to hasten ISIL's lasting defeat," Carter said in a statement, using another name for ISIS. "These additional U.S. forces will bring unique capabilities to the campaign and provide critical enabler support to Iraqi forces at a key moment in the fight."
Speaking to reporters in Baghdad, the defense secretary said, "It's necessary but not sufficient to destroy ISIL in Iraq and Syria because this is where it began and is what I have called the parent tumor of the cancer ... but, like cancer, ISIL has spread to ... other places and it also threatens our homelands."
MacFarland said, "We are starting to put pressure on the enemy's terror networks, threat networks that are around the capital, and we'll continue to ramp that up over time."
Rep. Mac Thornberry, chairman of the House Armed Service Committee, said in a statement after the additional troops were announced that he was "concerned that operational needs in Iraq and Syria are taking a back seat to troop levels the White House finds politically palatable."
"The war against ISIS and Islamic extremists cannot be won by inches," the Texas Republican said. "Added to the President's Afghanistan announcement last week, the United States will now be deploying thousands more troops than we have budgeted for in the President's budget request. Those deployments can only be fully supported through a supplemental budget request. I look forward to reviewing the President's request when he sends it to Congress, as I believe he now must."
Carter arrived in Baghdad Monday for a day-long visit to discuss the Iraqi army's plans to retake Mosul, which has been in ISIS' hands since June 2014.
On the flight to the Iraqi capital, Carter discussed the seizure of the Qarayyah airfield
, which was accomplished by Iraqi forces, backed by U.S.-led airstrikes, and announced Saturday by Iraqi Prime Minister Haider al-Abadi.
The recapture of Qarayyah, one of the biggest air bases in the country, is seen as a breakthrough in the mission to liberate Mosul, and comes just weeks after Iraq declared it had regained full control of Falluja
, ISIS' main stronghold, as the militant group continues to lose ground.
Iraqi officials said they will move the headquarters for the liberation of Mosul to Qarayyah and its airstrip will bring Iraqi and coalition aircraft that much closer to the city.
Carter also reiterated on the flight to Baghdad that U.S. forces would accompany an assault -- in an advisory capacity -- on Mosul, Iraq's second-largest city and the largest in Iraq under ISIS control.
"U.S. units are prepared to advise and accompany to the battalion level," Carter said. "All of that is part of the campaign plan that we've agreed to with Iraqis."
On his fifth trip to the Iraqi capital, Carter was greeted at his plane in Baghdad by U.S. Ambassador to Iraq Stuart Jones and MacFarland.
During a meeting later at the Prime Minister's office, Carter expressed condolences to the Iraqi people on behalf of the U.S. for the recent terror attacks in Iraq and said they strengthened his determination to fight ISIS.
Iraqi forced poised to recapture city
Iraqi Defense Minister Khalid Al-Ubaidi, a Mosul native, has declared that "2016 will be the year of the liberation of Mosul and the rest of Iraq."
Already, in just over a year, Iraq has driven ISIS out of the key cities of Tikrit, Ramadi and Falluja.
Iraqi and Kurdish forces have been training and preparing for the final battle.
A new "Nineveh Liberation Operations Center" has been set up to coordinate the offensive, complete with dozens of U.S. and British advisers. Nineveh is the province where Mosul is located. A U.S. artillery unit is also providing cover for operations south of Mosul.
Kurdish forces, or Peshmarga, are dug in to the east, north and west of Mosul, and Iraqi forces are moving slowly from the south. If previous experience is anything to go by, they'll probably encircle the city, clearing ISIS from the towns and villages around it before entering the city proper.