US-led coalition air strikes targeting Islamic State (IS) group militants in northeastern Syria killed at least 12 civilians from a single family, a Syrian activist group said Saturday.

"The air strikes and artillery fire (Friday night) by the international coalition on the village of Hidaj, held by IS in the southern sector of Hasakeh province, killed at least 12 people," the Britain-based Syrian Observatory for Human Rights said.

The Observatory said there have now been 20 civilians killed by coalition operations against IS near Syria's border with Iraq over 24 hours, after eight were killed in Deir Ezzor province on Thursday.

IS has lost most of the self-proclaimed "caliphate" it once controlled in large parts of Syria and neighbouring Iraq it took in 2014.

Today, it holds less than three percent of Syria, according to the Observatory.

In recent weeks the US-led coalition has been backing an offensive by the Kurdish-led Syrian Democratic Forces against remaining IS fighters on the east bank of the Euphrates River.

The coalition said Friday its air strikes in Syria and Iraq had "unintentionally" killed 892 civilians since its bombing campaign began nearly four years ago.

More than 350,000 people have been killed in Syria's war since it started in 2011 with a brutal crackdown on anti-government protests.

Syrian President Bashar al-Assad told Russia Today on Thursday he would seize back territory controlled by US-backed forces in Syria if negotiations failed.