Islamic State (IS) militants have attacked the northern Syrian town of Tal Abyad, which they lost to Kurdish fighters two weeks ago, activists say.
The Syrian Observatory for Human Rights said IS had seized a district of the town, which is on the Turkish border.
The loss of Tal Abyad was a significant blow to IS, as it lay on a key supply route to its stronghold of Raqqa.
In the past week, IS has tried to regain the initiative by launching a series of assaults in the region.
The most prominent saw a group of jihadists infiltrate Kobane, west of Tal Abyad, killing more than 200 people over the course of two days.
The Syrian Observatory, a UK-based monitoring group, said IS militants had entered Tal Abyad on Tuesday and taken control of the Mashor Foqani district, on the town's eastern outskirts.
Kurdish Popular Protection Units (YPG) fighters were now trying to encircle the jihadists and prevent them from making further advances, it added.
The YPG, backed by allied Syrian rebels and US-led coalition air strikes, captured Tal Abyad on 16 June, only days after launching a two-pronged assault on the town that prompted thousands of residents to flee across the border with Turkey.
Afterwards, the Kurds advanced southwards and overran the IS-held town of Ain Issa, only 50km (30 miles) from Raqqa.
The capture of Tal Abyad allowed the YPG to connect the Kurdish enclaves, or "cantons", surrounding Kobane and the city of Qamishli, east of Tal Abyad.