Clashes raged around a key northern Syrian town on Tuesday (March 28th) after the "Islamic State of Iraq and the Levant" (ISIL) launched a counter-attack to fend off a Syrian Democratic Forces (SDF) advance near the group's stronghold al-Raqa, AFP reported.

Backed by air power from the international coalition, the SDF are laying the groundwork for an assault on the heart of the extremists' so-called "caliphate".

A key part of the campaign is the battle for the ISIL-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 ISIL fighters doubled down on their defenses on Tuesday, the Syrian Observatory for Human Rights said.

"The fighting is a result of ISIL launching a counter-offensive to exhaust the Syrian Democratic Forces around the Tabqa military airport," said Observatory head Rami Abdel Rahman.

He said the SDF were working to "consolidate their 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 ISIL.

After a brief pause in fighting on Monday to allow technicians to enter the dam complex, SDF fighters resumed their operations around the structure, said spokeswoman Jihan Sheikh Ahmed.

"ISIL 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.

On Tuesday, technicians accompanied by the Syrian Arab Red Crescent could be seen examining the dam to assess whether water levels had risen in recent days.

"The explosions and the clashes are threatening the dam, and we ask for all sides to distance themselves from it," said Ismail Jassem, an engineer from the SDF-controlled Tishreen Dam in neighbouring Aleppo province.

"The water levels are acceptable now. We came to open up one of the gates to relieve the pressure," he said.