Islamic State (IS) militants are reported to be shoring up the defences of Raqqa, the northern Syrian city that acts as the group’s headquarters.
The move follows the capture on Tuesday by Kurdish fighters of a strategically -important town and nearby military base only 50km (30 miles) to the north.
The Kurds said on Wednesday that trenches were being dug around Raqqa.
The prices of basic supplies are also said to have risen in the city because key supply routes have been cut.
IS has suffered a string of defeats to the Kurds in areas along the Turkish border since being forced to withdraw from Kobane in January after a four-month battle.
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.
Ain Issa is situated at an intersection of the main roads from Raqqa – the de facto capital of the caliphate whose creation IS announced a year ago – to other IS-held areas in Aleppo province, to the west, and Hassakeh province, to the east.
The YPG has said it is not yet planning an assault on Raqqa, and is reportedly focusing on taking control of the east-west road passing through Ain Issa.
But on Wednesday, YPG spokesman Redur Xelil said it had received information that IS militants were preparing for an assault and had “begun digging trenches in the vicinity of Raqqa to improve their defences”, the Reuters news agency reported.
The Syrian Observatory for Human Rights, a UK-based monitoring group, also said a convoy of vehicles had been seen heading to the sprawling Division 17 military base and other areas just north of Raqqa to help fortify IS positions.
US officials told Reuters that the defeats inflicted on IS by the Kurds were the result of an increasing number of air strikes, intelligence sharing, and weapons air-drops.
But they also warned that IS remained resilient and capable of taking the initiative.
In May, the group responded to the loss of the northern Iraqi city of Tikrit and the launch of a government offensive in the western province of Anbar by overrunning the city of Ramadi, prompting 275,000 residents to flee.