The Syrian Democratic Forces (SDF) launched an operation to capture Raqa in northern Syria last November, and have since surrounded the city from the north and east while closing in from the west.
The Britain-based Syrian Observatory for Human Rights monitor said the SDF captured the town of Mansura late yesterday and the adjacent Baath Dam on the Euphrates River, around 20 kilometres west of Raqa today.
SDF spokesman Talal Sello confirmed the force was advancing on the western front without providing details.
"We're nearing the major battle," he added.
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Abdel Rahman said combing operations were ongoing in Mansura and at the dam to "dismantle mines and search for remaining jihadists".
Sello told AFP the advances on the western front were part of the final stage of operations before the launch of the assault on Raqa itself.
He said the SDF had received "weapons and advanced equipment from the international coalition... As part of preparations for the launch of the battle for Raqa, which is close".
"The SDF has already completed the siege from the northern and eastern sides and is working to complete the siege from the west," he added.
The SDF is fighting with broad support from the US-led coalition, which has provided air cover, assistance from special forces on the ground, and weaponry.
SDF fighters are now just a few kilometres from the city on the north, east and western fronts, with all major routes into the city severed.
IS fighters are still able to move out of the city to the south, however, crossing the Euphrates river by boat before continuing on through the desert.
"The SDF doesn't need to isolate Raqa from the south, because the international coalition's planes can target any jihadists as they cross the river," Abdel Rahman said.