BAGHDAD — The United States will send 560 more troops to Iraq to help establish a newly retaken air base as a staging hub for the long-awaited battle to recapture Mosul from Islamic State militants, Defense Secretary Ash Carter said Monday on an unannounced visit to the country.

Most of the new troops will be devoted to the build-up of the Qayara air base, about 40 miles south of Mosul, and include engineers, logistics personnel and other forces, Carter said. They will help Iraqi security forces planning to encircle and eventually retake the key city.

“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,” Carter said, according to prepared remarks.

He revealed President Barack Obama’s decision as he spoke to about 120 troops in a building at Baghdad’s airport, shielded from scorching desert hovering near 100 degrees Fahrenheit. Many were members of the 101st Airborne Division, known as the Screaming Eagles. The increase brings the total U.S. force authorization in Iraq to 4,647, and comes just three months after Obama’s last troop addition.

Carter told reporters earlier that U.S. advisers are prepared to accompany Iraqi battalions if needed, as those units begin the siege of the key northern city. It’s not clear when exactly that will happen. U.S. officials said a team of American troops went into Qayara for a quick site assessment Sunday and left.

One potential job is helping Iraqi troops use highly technical bridging capabilities to get across the river into Mosul.

Carter called this weekend’s recapture of Qayara a key strategic victory. Speaking to reporters before he arrived in Baghdad, he said the air base will be one a hub from which “Iraqi Security Forces, accompanied and advised by us as needed, will complete the southern-most envelopment of Mosul.