U.S. Defense Secretary Ash Carter arrived on an unannounced visit to Baghdad on Saturday to get an assessment of the U.S.-backed Iraqi campaign to retake Mosul from Islamic State.

"We will see the prime minister (Haider al-Abadi) and get his best assessment on where we are with the Mosul operations," a senior U.S. defense official said, briefing reporters ahead of the trip.

The offensive that started on Monday to capture Mosul is expected to become the biggest battle fought in Iraq since the U.S.-led invasion in 2003.

With air and ground support from the U.S.-led coalition, Iraqi government forces and Kurdish Peshmerga fighters captured about 50 villages south, east and north of Mosul, according to military statements.

"It's the beginning of the campaign. We do feel positively about how things have started off, particularly with the complicated nature of this operation," the official said.

Still, the United Nations says Mosul could require the biggest humanitarian relief operation in the world, with worst-case scenario forecasts of up to a million people being uprooted. About 1.5 million residents are still believed to be inside Mosul.

Beyond being briefed on the campaign itself, Carter is expected to hear about preparations for the stabilization of Mosul in the event that Islamic State is defeated.

The defense official said the briefings would focus on "day-after scenarios" in Mosul, including humanitarian assistance, stabilization efforts and planning on governance of the city.

"This is a recognition that for success in Mosul really means more than just the military part of the assault. It really means the follow up," the official said.

Mosul is the last big stronghold held by Islamic State in Iraq and about five times the size of any other city the group has held. The battle could stretch on for weeks or months, officials say.

On Friday, Carter signaled his support for a possible Turkish role in the campaign and said there was an agreement in principle between Baghdad and Ankara -- potentially ending a source of tension.

The details on any Turkish participation still needed to be worked out, he said.

FIRST US DEATH IN MOSUL CAMPAIGN

Roughly 5,000 U.S. personnel are in Iraq. More than 100 of them are embedded with Iraqi and Kurdish Peshmerga forces involved with the Mosul offensive, advising commanders and helping ensure coalition air power hits the right targets.

U.S. Navy Chief Petty Officer Jason Finan was killed on Thursday by a roadside bomb in northern Iraq as he was accompanying Iraqi forces, in the first U.S. casualty of the Mosul campaign.

The fall of Mosul would signal the defeat of the ultra-hardline Sunni jihadists in Iraq but could also lead to land grabs and sectarian bloodletting between groups that fought one another after the 2003 overthrow of Saddam Hussein.

U.S. military commanders expect Islamic State to put up a fight.

"ISIL knows the battle for Mosul is really the battle for Iraq for them...we are watching closely to see the level of effort they're going to put into repelling the attacks," the U.S. defense official said, using an acronym for Islamic State.

In a possible sign of Islamic State's tactics as Iraqi forces focus on Mosul, militants attacked the oil city of Kirkuk on Friday, killing at least 35 people.

By Reuters