"A cell of Islamic State fighters infiltrated Tal Abyad and took control of a district in the eastern outskirts of the town," said Rami Abdel Rahman, director of the Syrian Observatory for Human Rights.
"Kurdish fighters are trying to encircle the jihadists and prevent them from advancing further."
He said fighting was continuing in the town, which lies on the border with Turkey. Tal Abyad was a crucial IS supply hub and stronghold for around a year before Kurdish forces expelled the group.
He had no immediate toll from the clashes.
"Daesh uses this method of infiltration to enter areas it was previously expelled from," Khalil said, using an Arabic acronym for IS.
The YPG, backed by Syrian rebel allies, seized Tal Abyad on June 16, just days after beginning an advance against the town.
The ground forces were backed by air strikes from the US-led coalition fighting IS in Syria and Iraq.
The battle for Tal Abyad prompted tens of thousands of refugees to flee across the border into Turkey, with the influx at times causing chaos.
Kurdish forces have battled IS in several areas in the northern Syrian region along the border with Turkey.
In January, they secured the Kurdish border town of Kobane, which IS forces had been trying to capture for about four months.
Afterwards, the Kurds and their Arab allies began chipping away at IS territory in their stronghold province of Raqa, where the jihadist group's de facto Syrian capital is located.
But shortly after taking Tal Abyad from IS, the Kurds came under attack again in Kobane.
