Iraqi forces launched an offensive against the Islamic State group near the Syrian border Thursday, piling further pressure on the jihadists' crumbling "caliphate".

Baghdad and its allies also turned up the heat on IS in its last remaining Iraqi stronghold of Mosul, where the US-led coalition said it had doubled the number of its advisors.

"A military operation has begun in the western areas of Anbar (province) to liberate them from Daesh," said Lieutenant General Qassem Mohammedi, head of Jazeera Operations Command, using an Arabic acronym for IS.

He said the operation was led by the army's 7th division, police, and fighters from local tribes that have opposed the jihadists, with aerial backing from the coalition.

The main targets of the operation are Aanah, Rawa and Al-Qaim, the westernmost Iraqi towns along the Euphrates Valley.

The jihadist hub of Al-Qaim, which lies 330 kilometres (200 miles) northwest of Baghdad, is still a long way down the road and the most immediate target is the town of Aanah.

"Our forces started advancing from Haditha towards Aanah from several directions," Mohammedi told AFP.

Haditha was never seized by IS when the group swept across much of Iraq's Sunni Arab heartland in 2014 and is home to a tribe that has led the fight against the jihadists in the area.

"Zero hour has come to liberate the western areas," Nadhom al-Jughaifi, a commander with the Haditha tribal fighters, said.

In 2016, Iraqi forces retook large parts of the vast province of Anbar, including its capital Ramadi and the city of Fallujah.

Anbar is a desert area traversed by the Euphrates that borders Saudi Arabia, Jordan and Syria. Security in reconquered areas remains precarious and jihadists continue to move across the province.

- '200,000 buildings' -

IS has lost more than half of the territory it once controlled in Iraq and the loss of Mosul would deal a major blow to the "caliphate" it proclaimed there in June 2014.

Tens of thousands of Iraqi forces are currently involved in an offensive to retake the main northern city, which is also IS's last major stronghold in the country.

The operation launched on October 17 is Iraq's largest in years and while significant territory was reconquered around Mosul, the going has been tough inside the city itself.

After a lull in operations, Iraqi forces launched a fresh push last week and appear to have found new momentum.

"Iraqi security forces have made significant progress since initiating phase two of their operation to liberate Mosul," Colonel John Dorrian, the coalition's spokesman, said on Wednesday.

He said that was partly owed to increased coalition involvement in the battle, with a doubling of the deployment of advisors there to about 450.

"We have increased the number of advise and assist forces that are there with the ISF (Iraqi Security Forces) command elements to help advise them as they move forward and to synchronise operations," he said.

On Wednesday alone, coalition aircraft conducted seven strikes in the Mosul area, destroying a large number of IS targets, including five buildings, four mortar positions, four fighting positions, and damaging 27 supply routes.

Prime Minister Haider al-Abadi promised that his forces would rid Iraq of IS by the end of 2016 but commanders have admitted they were surprised at how stiff jihadist resistance was in the city.

According to a top commander in the Counter-Terrorism Service that has spearheaded the battle in Mosul, Iraqi forces have now retaken about two thirds of the city's eastern half.

Dorrian said the presence inside the city of hundreds of thousands of civilians had slowed progress.

"There are more than 200,000 buildings in Mosul. And really, in order to do this properly, given the way that the enemy has conducted themselves, you end up having to clear each one," he told reporters.