The Syrian Observatory for Human Rights said clashes were raging around Tabqa, a town on the southern bank of the Euphrates River.
"The fighting is a result of ISIS launching a counter-offensive to exhaust the Syrian Democratic Forces around the Tabqa military airport," said Observatory head Rami Abdel Rahman.
The battle for Tabqa is a major part of the SDF's flagship campaign for the city of Raqqa, the Syrian heart of ISIS's so-called "caliphate".
Backed by the US-led coalition bombing ISIS, the SDF seized the Tabqa airbase late Sunday and began pushing north.
Abdel Rahman said the SDF, an alliance of Kurdish and Arab forces, was working to "consolidate its positions" near the airport ahead of a final push for the town.
SDF fighters are also bearing down on the Tabqa dam after capturing its northern entrance on Friday from ISIS terrorists.
After a brief pause in fighting on Monday to allow technicians to enter the dam complex, SDF fighters resumed their operations around the structure, spokeswoman Jihan Sheikh Ahmed said.
"ISIS amassed its fighters and attacked our forces in the area, which forced us to respond and resume the operations to liberate the dam," she said.
Earlier this year, the United Nations raised concern about the prospect of damage to the dam in fighting, warning that water levels -- which put pressure on the structure -- were already high.
At their closest point, the SDF are just eight kilometres (five miles) from Raqqa city, to the northeast. But they are mostly further away, between 18 and 29 kilometres (between 11 and 18 miles) from Raqa.
The Britain-based Observatory, which relies on a network of sources on the ground across the country, said ISIS had deployed around 900 fighters from Raqqa city to various fronts in the wider province.
"Fighting is raging on every front around the city of Raqqa, accompanied by non-stop air strikes," Abdel Rahman said.