Kurdish-Arab forces launched a major assault against the Islamic State group in Syria's Raqa province on Tuesday and Iraqi forces advanced on it in Fallujah, piling pressure on the jihadists in two strongholds.

The twin offensives marked some of the most serious ground efforts against IS since the group declared its self-styled "caliphate" straddling the Syrian-Iraqi border in 2014.

Territory under IS control has been steadily shrinking for months but it has carried out a wave of attacks including bombings in the Syrian government's coastal heartland on Monday that killed 177 people.