Iraqi Prime Minister Haider al-Abadi on Saturday declared victory in a three-year war by Iraqi forces to expel the militant Islamic State (IS) group from the country.

“Our forces are in complete control of the Iraqi-Syrian border and I, therefore, announce the end of the war against Daesh (IS),” Abadi told a conference in Baghdad.

“Our enemy wanted to kill our civilisation, but we have won through our unity and our determination. We have triumphed in little time,” he said, hailing Iraq's “heroic armed forces”.

The IS seized vast areas north and west of Baghdad in a lightning offensive in 2014.

With Iraq's army and police retreating in disarray at the time, Ayatollah Ali Sistani, spiritual leader of the country's majority Shias, called for a general mobilisation, leading to the formation of Hashed al-Shaabi paramilitary units.

Iraq's fightback was also launched with the backing of an air campaign waged by a US-led coalition, recapturing town after town from the clutches of the militants in fierce urban warfare.

“Congratulations to the government of Iraq and the Iraqi security forces on the liberation of all Daesh-held populated areas in Iraq,” the coalition said in a statement on Twitter, using an Arabic acronym for IS.

However, Hisham al-Hashemi, an expert on militant groups, warned that the IS still posed a threat by retaining arms caches in uninhabited desert zones. Iraq's close ally Iran already declared victory over IS last month, as the militants clung to just a few remaining scraps of territory. But Abadi said at the time that he would not follow suit until the desert on the border with Syria had been cleared.

The militants' defeat is a massive turnaround for an organisation that in 2014 ruled over seven million people in a territory as large as Italy encompassing large parts of Syria and nearly a third of Iraq.

On the Syrian side of the border, the IS is under massive pressure too. On Thursday, Russia's defence ministry said its mission in support of the Syrian regime to oust IS militants had been “accomplished” and the country was “completely liberated”.

In the border region, pro-government forces and US-backed Kurdish-led forces are conducting operations to clear IS fighters from the countryside north of the Euphrates valley after ousting them from all Syrian towns.