Australians face the prospect of time behind bars for visiting a major stronghold of Islamic State.

New counter-terrorism laws mean Australians who visit Mosul in northern Iraq could face up to ten years behind bars.

Australian correspondent Donna Demaio says there will be exceptions for aid workers, journalists and people with relatives in the area.

"Julie Bishop has urged any Australians in the area illegitimately to leave immediately."

Meanwhile a massive operation has been launched by the Iraqi government to recapture Tikrit, a city occupied by Islamic State for the last eight months.

It's about 130 kilometres north of the capital, Baghdad.

CNN's Ben Wedeman says approaches are being made by forces of 30,000 fighters from the south, north and north-east.

"It involves around 30,000 Iraqi soldiers and policemen, but also Sunni tribesmen and Shia militiamen as well."

Ben Wedeman says already they've taken areas south of the city and claim to have inflicted heavy casualties.