The Syrian army has crossed the boundary of 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 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.

Heavy Russian air strikes hit Islamic State-held territory in eastern areas of Syria's Hama province, near the boundary of Raqqa province, on Friday when 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.

The army was making its advances from the Athriya area of eastern Hama province, close to the provincial border with Raqqa. 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.

State media has given no indication of how many troops are involved in the offensive, or what weaponry they might be using.

The Observatory also had no comment about numbers or weapons, but said at least 26 Islamic States militants had been killed along with nine from the Syrian and allied forces.

The war monitor said the army advance meant it was now almost 40 km from an area in which U.S. backed rebels were also waging an offensive to isolate the militants' strongholds in northern Aleppo from their territories east of the Euphrates river, where Raqqa city is located.

Should the army be able to reach the area where the rebels are also fighting Islamic State, that would leave the ultrahardline group hemmed in here - albeit by forces highly unlikely to work together as they are on opposing sides in the multi-faceted conflict.

The influential pro-Damascus Lebanese newspaper Al-Akhbar said on Friday the army operation did not aim to reach Raqqa city within the coming weeks, but was to reach Tabqa city and Lake Assad, which Taqba overlooks.

Islamic State captured Tabqa in 2014 at the height of its rapid expansion in Syria and Iraq. Tabqa, the location of an air base, is some 50 km (30 miles) west of Raqqa city.

The town is on a key route that links Raqqa with areas the ultra hardline militants control in northern Aleppo.