Iraq said it was closing its main border crossing with Jordan on Thursday until further notice, days after security forces and Shi'ite militias launched an offensive to reclaim the western province of Anbar from Islamic State militants.

An Iraqi army officer who saw a copy of an Interior Ministry letter sent to his brigade said authorities closed the Tureibil crossing to deprive Islamic State of funds raised from truck drivers forced to pay a tax on each cargo coming in from Jordan.

A triple suicide bombing at the Tureibil crossing in April was claimed by Islamic State, which controls most of Anbar and large tracts of northern Iraq, captured in a rapid offensive last summer.

Iraq's Shi'ite Muslim-led government on Monday announced the start of operations to "liberate Anbar", the province west of Baghdad whose Sunni Muslim cities and towns along the Euphrates have since last year become strongholds of Islamic State.

Islamic State militants seized control of the last government-held border crossing between Iraq and Syria in May, consolidating its self-proclaimed caliphate in parts of Syria and Iraq straddling the border.