Iraq's Army declared victory over Islamic State fighters in a provincial capital west of Baghdad, the first major triumph for the US-trained force since it collapsed in the face of an assault by the militants 18 months ago.

The capture of Ramadi, capital of mainly Sunni-Muslim Anbar province in the Euphrates River valley west of the capital, deprives Islamic State militants of their biggest prize of 2015. The fighters seized it in May after government troops fled in a defeat which prompted Washington to take a hard look at strategy in its ongoing air war 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. The next step is to clear pockets that could exist here or there in the city,” said Sabah al-Numani, a spokesman for the force leading the fight on the government side.

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.

A US defense official, speaking on condition of anonymity, said the US-led campaign against Islamic State was unable to confirm at this point whether the militants had been cleared out of the government complex.

Islamic State, also known by the acronyms ISIS, 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.

Its rise was aided by the swift collapse of the Iraqi Army, which abandoned city after city, leaving fleets of armored vehicles and other American weapons in the fighters’ hands.

A US-led coalition is waging an air campaign against Islamic State, but rebuilding the Iraqi Army to the point that it could recapture and hold territory has been one of the biggest challenges.

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

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.