The Syrian army has crossed the boundary of Raqqa province after advancing in a major Russian-backed offensive against Islamic State group militants, the Syrian Observatory for Human Rights, a group that monitors the war, said on Saturday.

Heavy Russian air strikes hit ISIS-held territory in eastern areas of Syria's Hama province, near the boundary of Raqqa province, on Friday and the army reached the edge of the province.

Raqqa city, further east, is Islamic State's de facto capital in Syria and, along with Mosul in Iraq, the ultimate target of those seeking to destroy the group's self-declared caliphate.

Syrian army spokesmen were not immediately available for comment.

The Syrian army offensive is the third big assault on the self-proclaimed caliphate in recent days after Iraqi forces attempted to storm a city and a Syrian militia advanced with U.S. support.

The three big offensives are some of the most aggressive campaigns against Islamic State since it declared its aim to rule over all Muslims from parts of Iraq and Syria two years ago.