ALBUQUERQUE — The United States 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 the Islamic State.

‘‘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 Ashton 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 Islamic State attacks.

There were 4,565 US 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 US 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 US 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.’’