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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 militants' stronghold of Raqqa.

Backed by air power from the international coalition bombing IS, the Syrian Democratic Forces (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 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 on Sunday and began moving north towards the town itself, but IS fighters began pushing back on Tuesday, said the Syrian Observatory for Human Rights. 

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

The SDF was working to "consolidate its positions" near the airport ahead of a final push for the town, he said.

IS launched the attack with a car bomb targeting SDF forces stationed near the airport, the alliance's media arm said.

Fierce clashes broke out but the SDF held back the attack and managed to seize some ammunition and rocket stores from IS, according to pictures published by the SDF.

SDF fighters are also bearing down on the Tabqa dam after capturing its northern entrance on Friday from IS fighters.

The fight around the structure has been backed by forces from the US-led coalition, with American-made armoured vehicles bearing the markings of the US Marine Corps seen moving along a nearby road.

'Acceptable' water levels

On Tuesday, coalition forces could be seen standing near military vehicles less than two kilometres (one mile) from the dam, their mortar rounds casually stacked nearby.

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 spokesperson Jihan Sheikh Ahmed.

"IS 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.

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.

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"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 told AFP.

IS has accused the US-led coalition of bringing the dam to near-collapse with its air strikes, and on Tuesday it said an American air raid killed top technicians at the dam.

"An American strike yesterday targeted the technical team at the Euphrates Dam, which killed the dam director, a technician, and an electrical engineer," the group said on its Amaq propaganda channel.

The SDF launched its offensive for Raqqa city in November, seizing around two thirds of the surrounding province, according to the Britain-based Observatory.

At their closest point, the forces 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 Raqqa.

The Observatory, which relies on a network of sources on the ground in Syria, said IS 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.

Syria's conflict began with protests against President Bashar al-Assad in 2011 but has turned into a brutal war pitting government forces, jihadists, rebels, and Kurds against each other.

UN-mediated talks between government and rebel representatives continued Tuesday in Geneva, aimed at bringing an end to the war that has killed 320,000 people.

The lead negotiator for the main opposition High Negotiations Committee, Mohamad Sabra, said in an interview late Monday that there could be "no real and viable political solution without the presence of the Americans."