A coalition of Iranian-backed Iraqi Shia militia plans to cross the border into Syria to fight alongside President Bashar al-Assad after "clearing" Islamic State militants from Iraq, a militia spokesman said on Saturday.

Iraqi Shia militiamen are already fighting on Assad's side in the country's civil war, and the coalition is currently participating in an Iraqi government offensive to recapture the northern city of Mosul from Islamic State. The announcement by the coalition, known as Hashid Shaabi, or Popular Mobilization, would formalise its involvement in Syria.

"After clearing all our land from these terrorist gangs, we are fully ready to go to any place that contains a threat to Iraqi national security," Ahmed al-Asadi, a spokesman for the Shia coalition, told a news conference in Baghdad.