Iraqi forces have closed in on Ramadi and launched an operation aimed at cutting off the jihadists in Anbar province before a major offensive to retake the city.

Ten days after the Islamic State group's shock capture of the capital of Iraq's largest province, a spokesman said on Tuesday that the latest operation was only a preparatory move before an assault on Ramadi.

A mix of security forces and paramilitaries are set to move south towards the city from Salaheddin province, Hashed al-Shaabi spokesman Ahmed al-Assadi said.

The Hashed al-Shaabi ("popular mobilisation" in Arabic) is an umbrella group for mostly Shi'ite militia and volunteers, which the government called in after the Islamic State group (IS) captured Ramadi on May 17.

"The operation's goal is to liberate those regions between Salaheddin and Anbar and try to isolate the province of Anbar," Assadi told AFP.

He said it had been dubbed "Operation Labaik ya Hussein", which roughly translates as "We are at your service, Hussein" and refers to one of the most revered imams in Shi'ite Islam.

The Hashed said 4000 men were heading to the northern edge of Ramadi.

Prime Minister Haider al-Abadi and his US allies had been reluctant to deploy Iran-backed Shiite militia in Anbar, a predominantly Sunni province.

Anbar's provincial capital had resisted IS assaults for more than a year but fell earlier this month after a massive jihadist offensive and a chaotic retreat by security forces.

The IS controls most of Anbar, a huge province which borders territory also under its control in neighbouring Syria.

Pockets of government control include some eastern areas near the capital, the city of Haditha, parts of the town of Al-Baghdadi and the Al-Asad air base, where hundreds of US military advisers are stationed.