Islamic State faces the final territorial defeat as the US-backed Syrian force battling the jihadists says it is closing in on their last bastion near the Iraqi border, capping four years of efforts to roll back the group.

Islamic State militants kills scores of people in a series of attacks on government-held parts of southwestern Syria, including multiple suicide blasts in Sweida city, official sources says.

Russia's air force has destroyed a large column of Islamic State fighters on their way to the Syrian city of Deir al-Zor, killing over 200 militants, Russian news agencies cites Russia's Defence Ministry as saying.