Beirut (AFP) - The international coalition battling the Islamic State group will begin a final push on the jihadists' Syrian stronghold Raqa in the coming days, French Defence Minister Jean-Yves Le Drian said Friday.

But on the ground, the US-backed Kurdish-Arab alliance spearheading the fight for the jihadist group's de facto Syrian capital expressed caution about how soon the battle for Raqa would begin.

IS has come under growing pressure from twin US-backed ground offensives targeting Raqa and its other main stronghold, Mosul in Iraq.

"Today, we can say that Raqa is surrounded and the battle will begin in the coming days," Le Drian told France's CNEWS television.

"This will be a very hard battle but essential."

The jihadists are under attack from several directions in northern Syria, with Russia supporting its Syrian ally President Bashar al-Assad on one front and Turkey providing air cover for rebel groups battling the jihadists on another.

The US-backed Syrian Democratic Forces (SDF), a Kurdish-Arab alliance, has been working for months to encircle Raqa.

But a spokesman for the alliance said there was still work to do.

"The operation to besiege Raqa will take several weeks and that will then lead to the official launch of the operation," Talal Sello told AFP.

- Strategic dam reached -

For now, the alliance is focused on the strategically important Tabqa Dam near Raqa and the adjacent town of Tabqa and its airport.

On Friday, they reached the dam's entrance, fighting IS in clashes that left jihadists dead and wounded, said Jihan Sheikh Ahmed, spokeswoman for the SDF's Raqa operation.

The US military has provided air and artillery support involving Apache helicopter gunships to help the SDF in the battle for the dam and the surrounding area, as well as airlifting in fighters.

"The operation is proceeding as expected on two fronts, the east and west," Ahmad told AFP.

"But the liberation of Raqa will take several months."

The United States has hundreds of troops on the ground in Syria supporting the SDF.

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But the alliance is still around eight kilometres (five miles) from Raqa at its closest point, to the northeast, and is mostly stationed further away, between 18 and 29 kilometres from the city, according to the Observatory.

A US official said last week that up to 1,000 additional American troops could deploy to northern Syria under provisional plans drawn up by the Pentagon.

- 'Complex situation' -

A European diplomat, who did not want to be named, said the situation surrounding the Raqa offensive remained "complex".

"The Americans are still in the review process," said the diplomat.

President Donald "Trump did not make a decision (on who will take Raqa), but it is clear that on the ground it is the SDF option that is developing."

The anti-IS coalition estimates that between 3,000 and 4,000 jihadists are in Raqa, a city of about 300,000.

Years of diplomatic efforts have failed to end the Syrian conflict, which has killed more than 320,000 people and displaced millions since it started in March 2011 with protests against Assad's regime.

The latest round of UN-backed Syria peace talks entered a second day in Geneva on Friday but there was little hope of a breakthrough.

Deadlock remains over most of the toughest issues, notably Assad's fate, with the opposition insisting he cede power and the government declaring the subject off limits.

The talks are meant to focus on a political transition, the constitution and elections, as well as counter-terrorism, at the request of Damascus.

UN mediator Steffan de Mistura, who has been meeting separately with the Syrian rivals, said Friday the two sides agreed to tackle all agenda items, including political transition.

"I am not expecting miracles, I am not expecting breakthroughs ... and I am not expecting breakdowns," he said.

Lead government negotiator Bashar al-Jaafari earlier said the talks should begin with the issue of "terrorism".

Rebels and allied jihadist have in recent days launched surprise offensives against government positions in central Hama province and the east of the capital Damascus.

On Friday, Syria's army command said the government had recaptured all the points lost during the offensive in east Damascus, which began Sunday from the Jobar neighbourhood, divided between regime and opposition control.