Transcript for Iraqi leaders declare victory over ISIS in Mosul

We want to turn to some news overseas. A milestone in Iraq, where u.s.-backed Iraqi forces are declaring victory over ISIS in mosul. Iraq's prime minister arriving to personally mark the end of a campaign to reclaim the city. But it still faces a humanitarian crisis, with ISIS likely to regroup and strike again. Here's Ian Pannell. Reporter: Tonight, a major victory against ISIS. The men who've bravely led this battle planting the flag in the name of Iraq and its people. Reclaiming mosul from ISIS. It's a victory, but at what cost? There's been little to cheer here for so long, that soldiers and civilians took to the streets to celebrate. Today, the prime minister came to mosul to declare the city liberated, congratulating troops for what he called a big victory. But the battle still isn't quite done here. Some ISIS fighters remain, holding out to the end. Endless bombing by the u.s.-led coalition, aimed at ISIS, has destroyed much of the old city. Thousands have died, many more injured. Held as human shields. Prisoners, starved in their own homes. Hard to imagine that Anas is 14 years old. His mother told me he used to be strong but he hasn't had food or water for ages. Ali was searching for his family. Their home was hit in an air strike weeks ago. He says he last spoke to his brother on the phone when he was trapped under the rubble, but then he stopped answering. So tonight, a celebration for Iraq. But a city that will never forgive or forget what ISIS did here. Now, all guns point westwards crucially to raqqa, Syria. The de facto capital for ISIS. There, u.s.-backed forces are already making headway against the militants. A long, hot and violent summer lies ahead. Tom? Ian, thank you.

This transcript has been automatically generated and may not be 100% accurate.