Islamic State's territory shrank by 40 percent from its maximum expansion in Iraq, and by 20 percent in Syria in 2015, as international forces pushed it out of several cities, the US-led coalition fighting it said on Tuesday.

There was no immediate comment from the hardline Islamist group on the estimates from the coalition, made up of countries including Britain, France and Jordan that have been bombing its positions.

"We believe in Iraq it's about 40 percent ... And Syria, harder to get a good number, we think it's around 20," coalition spokesman US Army Col. Steve Warren told a press briefing in Baghdad.

"Taking together Iraq and Syria .. they lost 30 percent of the territory they once held," he said.

Islamic State swept through a third of Iraq in 2014, seizing Mosul, the largest city in the north, and reaching the vicinity of Baghdad.

Counter-offensives by Iraqi and Kurdish armed forces supported by the US-led coalition, and by Iran-backed Shia militias have forced them out of several cities since, including Tikrit, north of Baghdad, and Ramadi, to the west last month.

In Syria, Islamic State is fighting the army of President Bashar al-Assad and other rebel groups opposed to his rule. It is facing air strikes by the US-led coalition and by Russia which has sent warplanes to support its ally, the Syrian government.

Iraqi Prime Minister Haider al-Abadi last month said 2016 will be the year of "final victory" on the hardline group.