Baghdad, Iraq (CNN) After retaking most of the key city of Ramadi from ISIS militants, Iraqi leaders say they are setting their sights on an even bigger prize: Mosul.

Iraqi forces have driven ISIS jihadists out of the heart of Ramadi, which the Sunni extremist group seized in May in a humiliating setback for the Iraqi government and the U.S.-led coalition that's backing it.

Significant pockets of ISIS resistance remain in Ramadi, still controlling as much as 25% of it as of Tuesday, local tribal leaders said. But that didn't stop Iraqi Prime Minister Haider al-Abadi from visiting the shattered city and raising the national flag.

Next year "will be the year we drive ISIS out of Iraq," he declared.

With efforts to finish clearing ISIS out of Ramadi still expected to take weeks, it seemed like a bold statement, especially in light of the militants' habit of hiding among local populations and booby-trapping territory they give up.

Differences between Mosul and Ramadi

Reclaiming Mosul, the country's second-largest city, will be a crucial step in trying to achieve Abadi's ambitious goal. CNN's Nima Elbagir, on her way to Ramadi, said it appeared that "very little" of the city's infrastructure remained.

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She said Ramadi had been retaken with the help of hundreds of airstrikes.

Mosul, a city of more than 1 million people about 400 kilometers (250 miles) north of Baghdad, fell to ISIS in June 2014 as Iraqi security forces fled en masse in a stunning defeat that underscored the Islamic militant group's rapidly expanding menace.

U.S. officials appear to be more cautious about the scale of the challenge than their jubilant Iraqi counterparts.

"Mosul is different than Ramadi, it's a big, big, big city, and it is going to take a lot of effort," said Col. Steve Warren, the spokesman for the U.S.-led coalition fighting ISIS. "It's going to take more training, it's going to take more equipment and it's going to take patience."

Squeezing ISIS out of Falluja

He expressed optimism, though, that Iraqi forces would be able to hold on to their gains in Ramadi and progress in efforts to retake nearby Falluja, another important city in the predominantly Sunni Muslim Anbar province.

"The fight for Falluja is ongoing. Right now, the Iraqi Security Forces are approaching Falluja from three directions," he said.

"Essentially, they encircle the city, almost like a boa constrictor, and they will then squeeze in closer and closer into that city until eventually they are able to finally clear it, as we saw in Ramadi," Warren told reporters.

Rebuilding a bombed-out city

The Iraqi military's success in Ramadi was notable for not relying on the Iranian-backed Shiite militias that were key to retaking the city of Tikrit from ISIS in March

Photos: What do you miss the most? Photos: What do you miss the most? CNN talked to internally displaced Sunni families in western Baghdad about the items they were forced to leave behind as they fled from ISIS brutality in Anbar province. Hide Caption 1 of 10 Photos: What do you miss the most? Yousif Hamed, 5, from Falluja – He misses his house and the plastic kiddy pool he used to play in. Hide Caption 2 of 10 Photos: What do you miss the most? Abdullah Mahmoud, 7, from Ramadi – Playing soccer with his friends, his kiddy pool, his bicycle and the games on his phone, all of which he had to leave behind. Hide Caption 3 of 10 Photos: What do you miss the most? Tabarak Mohammed, three-and-a-half, from Ramadi – The plastic cart that she used to push around, which was filled with toys. Hide Caption 4 of 10 Photos: What do you miss the most? Rasha Amer, 35, from Ramadi – Her sewing machine: Rasha is a seamstress and says she loves the work. Hide Caption 5 of 10 Photos: What do you miss the most? Yasemin Hakeem, 9, from Ramadi – Her clothes: Yasemin says her favorite items were a pair of black jeans and a black T-shirt, which her family didn't have time to grab as they fled. Hide Caption 6 of 10 Photos: What do you miss the most? Bilal Hamed, 19, from Falluja – His soccer ball. Bilal tells CNN that he just wants to be able to go back to the city and play soccer in the stadium with his friends. Hide Caption 7 of 10 Photos: What do you miss the most? Hassan Ali, 10, from Falluja – Laughing and playing with his friends -- as well as his bicycle and his Game Boy video game. Hide Caption 8 of 10 Photos: What do you miss the most? Yara Ali, 3, from Falluja – A red doll. Yara says she was unable to bring the toy because it was "sleeping" when her family fled. Hide Caption 9 of 10 Photos: What do you miss the most? Abdul Halim Ahmed, 35, from Ramadi – His motorcycle cart. Abdul says he used the cart to transport produce from one area to the other -- it was his means of making a living. Hide Caption 10 of 10

"There were no Shia militias involved in this operation for Ramadi," Warren said.

That approach was seen as vital to avoiding an escalation of the sectarian tensions that have helped ISIS' Sunni extremism gain such a stubborn foothold in parts of Iraq

Iraq's Shiite-dominated government in Baghdad is hoping that by bringing local Sunni tribal leaders in Anbar province into the political process, it can help prevent Ramadi and other areas from slipping back into ISIS' grasp.

Local officials will face a raft of challenges to rebuilding the ruined city, where the U.S.-led coalition says it has carried out more than 630 airstrikes in the campaign to retake it from ISIS since July.

The city, which tens of thousands of people fled during months of fighting, will need to restore basic infrastructure like electricity and running water, as well as its residents' sense of security.