US Defense Secretary Ash Carter made an unannounced visit to Iraq on Monday to meet US military commanders and Iraqi officials, as Washington seeks to capitalize on the recent seizure of a major airbase close to ISIS-held Mosul.

US and coalition forces will use the newly retaken air base in Qayara as a staging hub, as Iraqi security forces move forward in the long-awaited battle to recapture Mosul from ISIS militants, Carter said.

Carter said US advisers are prepared to accompany Iraqi battalions if needed, as those units begin to encircle the key northern city.

A senior defense official said it’s not clear when US advisers would begin accompanying the battalions closer to the battlefront. But it could be in the coming weeks and months. The US officials said a team of American troops went into Qayara for a quick site assessment Sunday and left.

One job they could do would be to help the Iraqis troops use highly technical bridging capabilities to get across the river into Mosul.

Carter laid out the US vision for Qayara for the first time, describing its recapture as a key strategic victory. Speaking to reporters before he arrived in Baghdad, he said the air base will be one of the hubs from which “Iraqi Security Forces, accompanied and advised by us as needed, will complete the southern-most envelopment of Mosul. That’s its strategic role, and that’s its strategic importance.”

Carter compared the role of Qayara to how forces used the eastern city of Makhmour. There, US troops set up a fire base for artillery to support advancing Iraqi units. Marine Staff Sgt. Louis F. Cardin was killed at the fire base in March in an ISIS rocket attack.

“The point of seizing that (Qayara) airfield is to be able to establish a logistics and air hub in the immediate vicinity of Mosul,” Carter told reporters. “So, there will be US logistics support.”

Iraqi forces recaptured the air base from ISIS on Saturday, in a victory hailed by Prime Minister Haider al-Abadi as a key step ahead of the Mosul fight. Residents of Mosul, Iraq’s second largest city, should “get ready for the liberation of their areas,” he said.

US officials said that American advisers have already been working at the brigade level with Iraqi special operations forces, but they have not yet accompanied Iraqi Army brigades, US officials said.

The officials were not authorized to discuss the matter publicly, so spoke on condition of anonymity.

In April, President Barack Obama approved plans to allow US troops to assist Iraqi forces at the brigade and battalion level, where they could be at greater risk, closer to the battle, but still behind the front lines. They had previously been limited to advising at the headquarters and division levels, which are further from the battle.

Carter met al-Abadi and minister of Defense Khalid al-Obeidi, as well as Lt. Gen. Sean MacFarland, the top US military commander for the ISIS fight. The main topic, he said, are the next steps in the military campaign, with a particular focus on Mosul.

Mosul is considered crucial. It was captured by ISIS in the summer of 2014 and the extremist group has been using it as a main headquarters since.

Carter’s daylong visit to Iraq comes on the heels of the two-day NATO summit where allies agreed to expand their military support for the war and approved several moves to expand military aid in the fight. NATO will use surveillance aircraft to collect intelligence, and will begin training Iraqi forces inside the country.

Until now, NATO has been training Iraqis in Jordan.