On Tuesday, Kurdish Popular Protection Units (YPG) militiamen, backed by allied Syrian rebels and US-led coalition air strikes, continued their rapid advance towards Raqqa by taking full control of the town of Ain Issa.
On Wednesday YPG spokesman Redur Xelil said in an online message the Kurds had received information that Islamic State had "begun digging trenches in the vicinity of Raqqa to improve their defenses" following the Kurdish advances.
The YPG said on Tuesday it was not yet planning an assault on Raqqa and the Syrian Observatory for Human Rights monitoring group has said the operations were aimed at seizing control of an east-west highway which links the city of Aleppo with the northeastern province of Al-Hasakah.
In a separate battle, the Syrian military and militias fighting alongside it have gained ground to the northwest of the city of Palmyra, which Islamic State captured from government control last month, according to the Observatory and a source in Syria briefed on the situation.
The YPG has emerged as the only notable partner on the ground to date for the U.S.-led alliance bombing Islamic State in Syria, and has fought several successful campaigns against the jihadists with air support.
"Our units also liberated the villages of Shiradi, Aghlik, Hoshin, Sherdik, Almnad, al-Inad, Masoura, Khalidiya, Sersuh, Oudiya, Awedana Oula and Awedana Thaniya", the Kurdish YPG leadership said.
The Islamic State has headquarters at Raqqa and YPG has been getting closer to it following continuous victories since they took over Kobane at the beginning of the year.
The YPG’s official Facebook page said "dozens of Daesh mercenaries were killed" at the Brigade 93 base. Arab countries in the region worry the Kurds are driving Arabs from some of the towns and cities they liberate in Syria, said Mustafa Alani, a security analyst at the Gulf Research Center based in Geneva. They are backed by Arab tribesmen, Assyrian Christian gunmen and members of the rebel faction known as Burkan al-Furat - Arabic for the "Volcano of the Euphrates".
"They are scared of Kurdish sleeper cells".
The success of the Kurds has raised the suspicions of the Turks, who fear they want to carve out their own state. The Iraqis have also suffered occasional losses. The IS group captured the capital of Anbar, Ramadi, last month. It was a repeat of the meltdown government forces suffered when IS militants swept across much of the nation’s north and west last summer.
Despite the Kurds’ recent gains, the Islamic State militants still have another supply line from Turkey that runs through northwestern Syria to Raqqa.
When cornered in the past, the militants have relied on coordinated mass suicide auto bomb attacks and other scorched-earth tactics.