Clashes raged around a key northern Syrian town on Tuesday after the Islamic State group launched a counter-attack to fend off a US-backed advance near the jihadists' stronghold Raqa.
Backed by air power from an international coalition bombing IS, the Syrian Democratic Forces are laying the groundwork for an assault on the heart of the jihadists' so-called "caliphate".
A key part of the campaign is the battle for the IS-held town of Tabqa on the Euphrates River, as well as the adjacent dam and military airport.
The SDF seized the Tabqa airbase late Sunday and began pushing north towards the town itself, but IS fighters doubled down on their defenses on Tuesday, the Syrian Observatory for Human Rights said.
"The fighting is a result of IS launching a counter-offensive to exhaust the Syrian Democratic Forces around the Tabqa military airport," said Observatory head Rami Abdel Rahman.