Islamic State’s de facto capital of Raqqa could become part of a new Kurdish-led state once it is freed from jihadist rule by American-backed forces, according to claims which will infuriate Turkey and much of the Syrian opposition.

The US-backed Syrian Defence Force (SDF) is moving in on Isil’s north-eastern base, leading to questions over what could take the place of Isil rule.

Unnamed Kurdish officials told the local ARA News that Raqqa, once freed, could either join a future federal region for northern Syria, or could have an independent local administration, similar to the local cantons that are now running the Kurdish areas of Syria.

Kurds in Syria are trying to carve out their own autonomous state, which would cover roughly 250 miles of territory along the Turkish border.

The SDF is a coalition forged by the US of the powerful Kurdish People’s Protection Units (YPG) with smaller Arab, Christian and Turkmen militias.

It is considered one of the most effective forces fighting Isil and has also gained the support of Russia. Both powers see it as the best chance for a multi-sectarian, non-Islamist Syria after the civil war.

Talal Silo, SDF spokesman, told the Telegraph it was gearing up for a “very difficult battle” for Raqqa.

The SDF, currently 15 miles away, is planning a “three-pronged attack” - surrounding the city from the northeast and northwest.

“There is already preparation to form a military council for Raqqa, once it is freed from Isil,” he said.

However, Rojava, as the Kurds call their autonomous zone in northern Syria, is deeply unpopular with both Turkey and other rebel groups.

For Turkey, the YPG is no more than the southern extension of the PKK, the guerrilla group which has been fighting Ankara for decades.

Meanwhile, the YPG has also been fighting not just Isil but other rebel groups north of Aleppo, some because they oppose the break-up of Syria, others because their Islamist ideology is contrary to its socialist, secular outlook.

Earlier this year, in an interview with the Middle East-focused website Al-Monitor, Abdul Karim Omar, a senior official, said the “Federal Democratic System of Rojava and Northern Syria” would have a population of four million, with self-governing Assyrian and Arab parts.

He denied the plan had been given a “green light” by the United States or Russia, but did suggest it could be a model for all of Syria.

It is a model which might also fit the outcome of the current partial ceasefire, which has left Syria broken up into separate fiefdoms.

In the last few weeks the SDF has been making gains on Isil, capturing one of the group’s main supply routes through north-east Syria.

Meanwhile, those still in Raqqa report that ISil fighters, who had been dug-in over the last two years, have begun repositioning in anticipation of the offensive.

US military said the movement could give overhead surveillance aircraft an improved chance of targeting them.

“We have seen this declaration of emergency in Raqqa, we know this enemy feels threatened, as they should,” Col. Steve Warren, spokesman for the US-led coalition, said.

“Isil understands that their days are increasingly numbered. We are going to continue to keep this pressure on them, and we expect to see them collapse eventually,” he added.