Kurdish militia made headway on Sunday into Syria's northern Raqa province, where the extremist Islamic State group has set up its de facto capital, Kurdish officials and a monitor said.
The Kurdish People's Protection Units (YPG) "took over a large cement factory and three villages" northwest of the town of Ain Issa, said the Syrian Observatory for Human Rights.
Observatory head Rami Abdel Rahman said the YPG had fought "fierce clashes" against IS jihadists in the past 48 hours and were now in control of a total of villages in the area.
At least six IS fighters had been killed in Sunday's clashes, bringing to 20 the death toll for the extremist group during the past two days, Abdel Rahman told AFP.
There was no immediate information on Kurdish casualties.
Ain Issa is less than 60 kilometres (35 miles) from the capital of IS's self-styled "caliphate" in Raqa city.
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It also lies just 45 kilometres south of Tal Abyad, a border town used by IS as a gateway for fighters coming in from Turkey.
A spokesman for the Kurdish Democratic Union Party (PYD) said YPG forces had taken over "a very large cement factory" in Raqa province.
"The YPG have been trying to take this area for a long time," Newaf Khalil told AFP by phone.
Since driving IS out of the flashpoint town of Kobane in January, Kurdish and allied forces have taken much of the surrounding countryside in the northern province of Aleppo and begun pushing east into neighbouring Raqa province.
In February, Kurds and rebel groups with air support from the US-led coalition against IS seized control of 19 villages in Raqa province.
YPG forces have also been pressuring IS in northeastern Syria, where they launched a counter-offensive against an IS assault.
On Saturday, they pushed IS back from Tal Tamr, a strategic town on a road running east across the Iraqi border to IS's bastion in Mosul, as well as north to the Turkish border.