The Syrian regime’s army, for the first time since 2014, crossed the boundary of Raqqa province on Saturday after advancing in a major Russian-backed offensive against ISIS, the Syrian Observatory for Human Rights said.

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.

“Regime troops backed by Russian air strikes and Russian-trained militia entered Raqqa province on Saturday morning for the first time since August 2014,” Observatory director Rami Abdel Rahman told Agence France Presse.

At least 26 jihadists and nine government troops and militia were killed in the advance, according to the Britain-based Observatory, which relies on reports from medics and activists on the ground.

Raqqa city, further east, is ISIS’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.

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

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