(CNN) Defense Secretary Ash Carter made a surprise visit to a military airfield near Mosul on Sunday, becoming the highest-ranking US official to visit the outskirts of the city where Iraqi troops are waging an intense fight with ISIS.

Qayyarah West, known as Q West, is home to US military advisers and is the closest position of US troops to the Mosul offensive.

A senior US defense official described the base as the "springboard for the campaign on Mosul."

Military analysts say Q West and its airstrip are vital to the effort to recapture Mosul because of their proximity to the city, the second largest in Iraq.

Hundreds of US military advisers are based at Q West, where they are assisting the advance of Iraqi troops. The base is also where the Iraqi government exerts command over the forces participating in the Mosul operation.

"It is critically important to the Iraqi Security Forces' success during the Mosul operation to have a logistical hub so close to Mosul," Patrick Martin of the Washington-based Institute for the Study of War told CNN.

Carter said he was visiting Iraq to thank the US, coalition, and Iraqi troops participating in the fight against the terror network and he will "survey key locations directly supporting the battle for Mosul," Pentagon press secretary Peter Cook said in a statement.

Iraqi Prime Minister Haider Al-Abadi, Kurdistan Regional Government President Masoud Barzani and Carter will discuss the next moves in the fight.

The defense secretary also planned to meet with members of the Iraqi security forces and the Kurdish Peshmerga, the fighters on the front lines in the battle, and with coalition and US. military personnel.

Mosul fight still intense

Carter told reporters aboard his aircraft that it was "certainly possible" that Mosul, the largest city still held by the terrorist group, could fall before the end of President Barack Obama's tenure on January 20.

But Carter acknowledged the fight for Mosul has been an intense one, with heavy casualties reported on both sides. The US-led coalition said it had conducted airstrikes near Mosul over the past few days and Iraqi and Kurdish forces have pushed into eastern Mosul, where US defense officials have said they have encountered some of ISIS' best fighters.

Some observers expect the fight to get even tougher as the battle moves to the more densely populated western Mosul, where the old city's narrow streets and alleys may favor defenders.

The visit comes the day after Carter announced that the United States would deploy an additional 200 troops to Syria in order to aid America's Arab and Kurdish allies as they continue their assault on ISIS' other major stronghold, its self-declared capital in Raqqa.

Iraq's 'will to fight'

A senior defense official told CNN one of the primary reasons for Carter's visit to Iraq was to thank not just US and coalition troops but also to praise the Iraqi military for its performance.

"I think we've all been kind of in awe of the sacrifices they've made and in how hard they have fought and we've taken a lot of pride" in that, the official added.

The laurels being bestowed by Carter mark a major departure from his view of the Iraqi military upon taking office in 2014. In his first interview as defense secretary, Carter told CNN's Barbara Starr that he questioned whether the Iraqis had the will to fight in the face of ISIS' dramatic battlefield gains.

The change in attitude "was brought about by the intensive training of Iraqi forces by the US and its partners, which increased Iraqi capacity, and by close embedding of US advisers with Iraqi units," Michael Knights, an expert on Iraq military affairs at the Washington Institute for Near East Policy, told CNN.

"Since the damaging 'will to fight' comments the Iraqi Security Forces have recaptured Tikrit, Bayji, Ramadi, Hit, Fallujah and Qayyarah," he added.

There are some 5,000 US troops in Iraq, and Carter told an audience in California last week that some of those forces would remain in Iraq after ISIS is defeated.

Regional security summit

On Saturday, Carter spoke at the Manama Dialogue, a regional security summit hosted in the capital of the Gulf state of Bahrain.

He said the United States would send up to 200 additional troops to Syria to help train and assist US-backed local forces that are driving towards ISIS' self-declared capital in Raqqa.

The new contingent would include special operations forces, trainers, advisers, and explosive ordinance disposal teams, he said.

"This latest commitment of additional forces within Syria is another important step in enabling our partners to deal ISIL a lasting defeat," Carter said, using the government's preferred acronym for the terror organization commonly known as ISIS.

Carter also stopped in Japan, India and Afghanistan. He also plans to visit Israel, Italy and the United Kingdom, Cook said in the statement.

The UK trip is the last stop of his journey. He plans to take part in a meeting of defense ministers from leading members of the coalition to vanquish ISIS.