Syrian troops pushed into the Islamic State group's bastion province Raqa on Saturday for the first time since 2014, in an advance towards the country's largest dam, a monitor said.
The Tabqa dam on the Euphrates River, 40 kilometres (25 miles) upstream from Raqa city, is also the target of a separate offensive launched by US-backed Kurdish-led forces advancing from the north late last month.
"Regime troops backed by Russian air strikes and Russian-trained militia entered Raqa province on Saturday morning for the first time since August 2014,", Syrian Observatory for Human Rights director Rami Abdel Rahman told AFP.
The lightning advance from the southwest brought government troops to within less than 40 kilometres (25 miles) of Tabqa, which is also the site of an airbase, Abdel Rahman said.
"It seems there has been an undeclared coordination between Washington and Moscow," he added, referring to the pincer movement by the two governments' respective Syria allies.
At least 26 militants 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.
Tabqa dam has a huge reservoir named Lake Assad after President Bashar al-Assad's late father and predecessor Hafez.
When IS group overran the area in 2014, it summarily executed 160 captured regime troops.
The militants are facing counter-attacks on multiple fronts.
Arab and Kurdish fighters backed by Washington have launched an assault on the strategic Manbij pocket further up the Euphrates on the Turkish border, regarded as a key entry point for foreign militants.
Hundreds of kilometres (miles) downstream in neighbouring Iraq, elite Iraq troops have launched an assault on the emblematic IS group bastion of Fallujah.