ALBUQUERQUE, New Mexico (AP) — The U.S. is sending 615 more troops to Iraq as the stage is set for an Iraqi-led battle to reclaim Mosul, the northern city that has been the Islamic State group's main stronghold for more than two years. The offensive, starting as soon as October, looms as a decisive moment for Iraq and for President Barack Obama's much-criticized strategy to defeat IS.

"These forces will be primarily to enable Iraqi security forces and also (Kurdish) Peshmerga in the operations to isolate and collapse ISIL's control over Mosul, but also to protect and expand Iraqi security forces' gains elsewhere in Iraq," Defense Secretary Ash Carter told reporters Wednesday. The Peshmerga are Kurdish militia fighters who are generally among the most proficient ground forces in Iraq but whose role is politically sensitive there.

Carter said the extra Americans would perform multiple roles at multiple locations, including at Qaraya West air base south of Mosul, where they will be building up the base to make it a hub for Iraq forces, and at al-Asad air base in Anbar province more than 200 miles away, where they will strengthen supply lines for the movement of supplies north toward Mosul.

Obama approved the deployment, which a senior defense official said would total 615 troops. Although the Americans are not to participate directly in combat, they may in some cases move forward with Iraqi combat forces and could face IS attacks.

There were 4,565 U.S. forces in Iraq as of Wednesday, according to the Pentagon. That number does not include as many as 1,500 troops who are there on temporary duty or are not counted for other bookkeeping reasons.

Carter underscored the potential risks to all U.S. troops involved in the campaign.

"We're in a support role, but I need to make clear once again: American forces combatting ISIL in Iraq are in harm's way," he said. "No one should be in any doubt about that."

Iraqi Prime Minister Haider al-Abadi, in a statement posted on his official website, said Wednesday the extra U.S. troops would "provide support for security forces and the Iraqi heroes in the fight looming in the liberation of Mosul." He said the Obama administration had approved his government's request for the increase.

Emphasizing that the Americans are there as advisers, Abadi added: "It is our troops who will liberate the land."

Abadi's last point is central to the U.S.-Iraqi strategy for delivering what Carter has called "a lasting defeat" to the Islamic State. Although some in the U.S. have urged Obama to put larger numbers of American combat troops on the ground in Iraq to defeat IS quickly, the administration has argued that a victory on those terms would be short-lived. They assert that Iraq must muster the will and cohesiveness - militarily and politically - to defend its own territory once the Islamic State has been pushed out.

For Obama, Iraq is not the only important battlefield. He faces even greater uncertainty and a dimmer outlook in neighboring Syria. The Islamic State has lost territory in Syria over the past year, but the U.S. has fewer reliable partner forces on the ground there. Even amid preparations to retake Mosul, the U.S. is trying to fashion an alliance of Syrian Arab and Kurdish fighters to assault Raqqa, the militants' main stronghold in Syria. The U.S. has only about 300 troops on the ground in Syria as advisers.

Mosul, the second-largest city in Iraq, will be a key test, but a victory there is unlikely to mean an end to Iraq's troubles. In a post-Islamic State Iraq, the enmities and rivalries among the players in the anti-IS coalition could easily erupt. The Iraqis have assembled a fragile alliance — Iraqi troops alongside Shiite militiamen, Sunni Arab tribesmen, Kurdish fighters and U.S special forces.

Although some Western officials have pointed to October as the likely start of a Mosul offensive, it's unclear whether Iraq will be ready by then. Carter said on Wednesday, "We are on schedule in terms of marshalling the force there," suggesting they are ready, or nearly ready.

Asked whether the extra 615 U.S. troops will be the final addition before the Mosul campaign kicks off, Carter said, "This is what we now foresee as required for the envelopment and seizure of Mosul. He said U.S. forces would remain for an undetermined period to help the Iraqi government consolidate its control over Mosul after the anticipated successful offensive.

U.S.-led coalition forces recently sped up training for Iraqi troops and Kurdish fighters, condensing courses that once took more than two months into just four weeks. In July, the Pentagon announced that 560 more U.S. troops would deploy to Iraq to transform Qayara air base, west of the city of Qayara and south of Mosul, into a staging hub for the final assault.

Obama has deliberately kept a lid on the U.S. troop presence in Iraq and ruled out using substantial ground combat units to recreate the enormous offensive firepower that Washington used during the 2003-2011 U.S. war in Iraq.

Carter said that while he is confident of victory in Mosul, it is hard to predict how costly it will be.

"We do not know what ISIL's plans will be for the defense of Mosul, nor whether they will be able to carry out whatever plans they have - whether their fighters will stick with them ... and so we're prepared for whatever happens there," he said.

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Associated Press writers Josh Lederman and Vivian Salama contributed to this report.

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Reach Josh Lederman on Twitter at http://twitter.com/joshledermanAP