Iraqi security forces and paramilitary units launched a dawn assault Tuesday against one of the Islamic State (IS) militants group's last bastions in the vast western province of Anbar bordering Syria.

The Joint Operations Command, which is coordinating Iraqi security forces battling IS, said an offensive had begun to retake the town of Anna and the nearby village of Al-Rayhanna.

"Infantry units and armour backed by the Hashed al-Shaabi began an offensive to liberate Anna and Al-Rayhanna from Daesh terrorists," JOC head General Abdelamir Yarallah said in a statement, using an Arabic name for IS.

A general in the area said the operation was "developing along three axes" involving the army, police and the Hashed al-Shaabi, a paramilitary force composed largely of Iran-trained Shiia militias.

The operation was supported by Iraqi army helicopters and warplanes from the US-led coalition battling IS in Iraq and Syria, the general said, speaking on condition of anonymity.

The head of Anna's municipal council, Abdel Karim al-Ani, confirmed the start of the offensive and said security forces had opened a road out of the town to allow civilians to flee.

Anna, about 100 kilometres (60 miles) west of the border with Syria, is one of three towns in Anbar province under IS control.

After retaking the town, Iraqi forces are expected to next target Rawa to the northwest and finally Al-Qaim, which is close to the border with the Syrian province of Deir Ezzor.

Three years after IS took control of large parts of Syria and Iraq, Iraqi forces are pushing to retake all areas seized by the militants, scoring their biggest victory in July with the recapture of second city Mosul.

Iraq is preparing to launch an assault against another of the militants' final strongholds, the town of Hawija about 300 kilometres (185 miles) north of Baghdad.

IS is also under pressure in Syria, where a US-backed alliance of Kurds and Arabs is battling to retake the militants' de facto capital Raqa and Russian-backed regime forces are fighting for control of the city of Deir Ezzor.