The Syrian army has crossed the boundary of Raqqa province after advancing in a major Russian-backed offensive against the Islamic State of Iraq and the Levant (ISIL) group, according to the Syrian Observatory for Human Rights.

Heavy Russian air strikes hit ISIL-held territory in eastern areas of Hama province, near the boundary of Raqqa, on Friday to facilitate the Syrian army's advance, the UK-based observatory said on Saturday.

ISIL, which controls large swaths of territory in Syria and Iraq, is fighting Syrian troops, US-backed fighters and other rebel groups in northern Syria and is facing an offensive by Iraqi government forces and its on their stronghold of Fallujah.

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"Regime troops backed by Russian air strikes and Russian-trained militia entered Raqqa province on Saturday morning," Rami Abdel Rahman, the observatory's director, told the AFP news agency.

The observatory said that at least 26 ISIL fighters and nine Syrian government and allied troops were killed in the fighting.

It was the first time that government troops had entered Raqqa province since they were ousted by ISIL fighters in August 2014.

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

The offensive brought troops to within less than 40km of Tabqa, which is the site of an airbase and a big reservoir, the observatory said.

The Tabqa dam on the Euphrates, 40km upstream from Raqqa city, is also the target of a separate offensive launched by US-backed Kurdish-led forces advancing from the north late last month.

Raqqa city, further east, is ISIL's de facto capital in Syria and also, along with Mosul in Iraq, the ultimate target of the international anti-ISIL coalition seeking to destroy the group's self-declared caliphate.

Al Jazeera's Hashem Ahelbarra, reporting from Gaziantep, on the Turkish side of the Turkey-Syria border, said that the push into Raqqa from was a "significant development" but there was a still "a long way to go".

"Raqqa is besieged by government troops from the west and Syrian factions from the north and from the east - the south is linked to the bordering province of Deir Az Zor which is still an ISIL stronghold," Ahelbarra said.

"Now, whoever controls Raqqa will face a mammoth challenge, which is basically securing a vast territory," he added, noting that both the Syrian government and the Kurdish factions cannot maintain a significant presence of troops in the province as they are also involved in heavy fighting in different battlefields elsewhere in Syria.