Iraqi security forces gather to advance towards the centre of Ramadi city, December 25, 2015.

Iraq’s army said on Sunday it had defeated Islamic State of Iraq and Syria (ISIS) fighters in a provincial capital west of Baghdad, the first major victory for the U.S.-trained force since it collapsed in the face of an assault by the militants 18 months ago.

Victory in Ramadi, capital of mainly Sunni-Muslim Anbar province in the Euphrates River valley west of the capital, deprives ISIS militants of their biggest prize of 2015.

The fighters captured it in May after government troops fled in a defeat which prompted Washington to take a hard look at strategy against the militants.

After encircling the city for weeks, the Iraqi military launched a campaign to retake it last week, and made a final push to seize the central administration complex on Sunday.

“By controlling the complex this means that we have defeated them in Ramadi,” said Sabah al-Numani, a spokesman for the force leading the fight on the government side. “The next step is to clear pockets that could exist here or there in the city.”

State television broadcast footage of troops, Humvee vehicles and tanks advancing through Ramadi streets amid piles of rubble and collapsed houses. Some districts appeared to have been completely destroyed by the advance.

Television also showed nighttime celebrations in mainly Shi’ite cities south of Baghdad, for the victory in Anbar, with people dancing in the streets and waving Iraqi flags from cars.

Officials did not give any immediate death tolls for the battle. The government says most civilian residents of the city were able to evacuate before it launched its assault.

ISIS, also known as ISIL or Daesh, swept through a third of Iraq in June 2014 and declared a “caliphate” to rule over all Muslims from territory in both Iraq and Syria, carrying out mass killings and imposing a draconian form of Islam.

Since then, the battle against the group in both countries has drawn in most global and regional powers, often with competing allies on the ground in complex multi-sided civil wars.

A U.S.-led coalition has been waging an air campaign against the fighters in both countries, but rebuilding the Iraqi army to the point that it was capable of recapturing and holding territory has been one of the biggest challenges.

In previous battles, including the recapture of former dictator Saddam Hussein’s home city Tirkit in April, the Iraqi government relied on Iran-backed Shi’ite militias for ground fighting, with its own army in a supporting role.

Ramadi was recaptured by the army itself, without relying on the militias, who were kept off the battlefield to avoid sectarian tensions with the mainly Sunni population.

“The complex is under our complete control, there is no presence whatsoever of Daesh fighters in the complex,” Numani told Reuters.

The government said the next target after Ramadi will be the northern city of Mosul, by far the largest population center controlled by Islamic State in either Iraq or Syria.

“The smooth victory in Ramadi should be happy news for the residents of Mosul,” Numani said.

U.S. officials had hoped Baghdad would launch an assault on Mosul during 2015, but this was put off after the fighters swept into Ramadi.

Dislodging the militants from Mosul, which had a pre-war population close to 2 million, would effectively abolish their state structure in Iraq and deprive them of a major source of funding, which comes partly from oil and partly from fees and taxes on residents.

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