The Syrian army pushed into Raqqa province, home to the de facto capital of Islamic State, after a major Russian-backed offensive against the militants, the Syrian Observatory for Human Rights said on Saturday.
The offensive is the third big assault on the self-proclaimed caliphate in recent days after Iraqi forces attempted to storm Falluja in central Syria and a Syrian militia advanced with U.S. support towards Manbij in the north near the Turkish border.
The 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.
Friday's assault saw the army reach the edge of Syria's Raqqa province after heavy Russian air strikes hit Islamic State-held territory in eastern areas of neighboring Hama 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.
State media said on Friday the army had made territorial gains and inflicted heavy casualties on the militants.
Syrian army spokesmen were not immediately available for comment.