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 Iraq 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 neighbouring 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.

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 ultra hard line group hemmed in, albeit by forces highly unlikely to work together as they are on opposing sides in the multi-faceted conflict.

ALEPPO OFFENSIVE

The U.S.-backed rebels also continued to make rapid advances in an offensive against IS-held areas in Aleppo province, beginning with the Manbij area where they continued to seize more territory, according to Kurdish sources and the monitor.

That thrust, supported by U.S. special forces, aims to deny Islamic State any access to the Turkish frontier, which is crucial for supplies of arms and food.

The Observatory said that these forces were able to reach nearly 5 to 6 km from Manbij town, further tightening the noose around the militants by cutting the town's main supply routes with Raqqa and laying siege to their fighters dug in the city.

"We made big progress and we are trying to ensure the safety of civilians before we begin our assault on the town," Sharfan Darweesh, a spokesman for the Military Council for Manbij, a tribal group affiliated with the U.S. backed Syria Democratic Forces (SDF) was quoted as saying.

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.

Separately, militants from radical Islamic groups led by al-Qaeda offshoot Nusra Front consolidated gains in the last 24 hours in southern Aleppo, according to rebel groups.

More than 1,000 militants have begun an offensive against Syrian army positions southwest of Aleppo, the Russian ceasefire monitoring centre in Syria said in a statement on Saturday.

The centre also reported civilians in Aleppo as saying armed groups partly made up of Turkish soldiers had appeared north of the city.

The insurgent advance will make it more difficult for the army and its allies to encircle rebel-held parts of Aleppo city where intensified bombing in the last 24 hours has killed scores of civilians mainly by barrel bombing of residential areas.

An attack last month by Nusra Front in southern Aleppo delivered one of the biggest battlefield setbacks yet to a coalition of foreign Shi'ite fighters, including Iranians and the Lebanese Hezbollah fighting in support of Syrian government forces.

It was a major success in recent months for rebels battling President Bashar al-Assad. – Reuters