Iraqi Prime Minister Haider al-Abadi tweeted Sunday that he has arrived in the "liberated city of Mosul" and congratulated "the heroic fighters and the Iraqi people in achieving this great victory."

However, state TV reported Sunday that ISIS militants are still holding out in one neighborhood of Mosul.

Mosul is Iraq's second-largest metropolis, and gaining control of the city was one of ISIS's most significant strategic wins.

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When the terror group seized Mosul in June 2014, it also took control of more than 2.5 million people and subjected some to horrors.