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After the anticipated Iraqi recapture of Mosul, the world lens will focus on the Islamic State group's last remaining hub in neighbouring Syria that will mark the fall of the militant's self-proclaimed "caliphate".

The US-led coalition has been striking IS across both countries for more than two years and has long held Raqqa in its sights - an IS defeat there, it is understood, would be a defining endpoint of the campaign.

But Pentagon chief Ashton Carter surprised commanders last week when he declared the actual assault on Raqqa could start "in the next few weeks", reiterating similar comments by UK Defence Minister Michael Fallon who made a similar prediction previously.

Privately, many high-ranking Pentagon officials were caught off-guard, and expressed scepticism that the Raqqa push could start so soon, given the Gordian knot of unpredictability in the chaos of Syria.

Carter's timeline is "a little more ahead of what I have been hearing so far," one US defence official told AFP on condition of anonymity, choosing his words carefully.

Another US defence official said the military's expectations for Raqqa - a city with a pre-war population of about 220,000 - didn't match Carter's timeline.

And a third military official said that while the assault could theoretically start in weeks, it may take "single-digit months", pushing the possible timeline out to "nine months or less".

"There are going to be more people who want to join the effort to dislodge IS" - Peter Cook, Pentagon press secretary

That official added the assault may start before 2017 "but it could drag on further for other reasons we can't control".

That is up to the local forces, he added.

"We are ready if they are ready."

Even before the start of a ground offensive - likely to be fought along similar lines to the one happening in Mosul - coalition planes must complete the "isolation", "shaping" and "envelopment" of Raqqa.

This entails non-stop strikes on IS fighting positions and the slicing of supply lines into and out of the northern Syrian city.

Coalition spokesman Colonel John Dorrian said those operations had already partially succeeded, and had helped cut routes from Raqqa to and from Europe.

"What we're talking about is a higher level of isolation that greatly reduces the freedom of movement of [IS] to go into and come out of that city," he said.

Unknown factors

Military officials are grappling with a slew of unknowns that have not been at play in the Mosul fight.

Whereas Iraqi security forces are a mostly cohesive fighting force under centralised control, the US-led coalition is relying on a more nebulous fighting crew in Syria.

The Syrian Democratic Forces (SDF) number around 30,000 fighters, two thirds of whom are Kurds fighting under the banner of the Kurdish People's Protection Units (YPG) with Syrian Arabs broadly making up the rest.

Some among the SDF's ranks are hardcore fighters, while others are newcomers to the conflict.

The coalition has spent months instructing SDF members, including on how to call in airstrikes, but the Pentagon admits some Arab fighters still haven't completed the weeks-long training.

Meanwhile, the ambiguity surrounding who will conduct the Raqqa offensive - US special forces will only be watching from behind the frontlines - was barely addressed this week by Pentagon press secretary Peter Cook.

He said the number of US-backed Syrian fighters would "snowball" as operations advance.

"There are going to be more people who want to join the effort to dislodge" IS, he said without going into details, alluding to complications surrounding the Turkish eagerness to contribute.

Torn alliances

While the US views the YPG as its best proxy fighting force in Syria, Turkey considers it a terrorist offshoot of the Kurdistan Workers Party (PKK) which has been waging an insurgency inside Turkey since 1984.

NATO member Turkey supports the recapture of Raqqa, but wants to limit Kurdish influence and prevent Kurdish cantons on either side of the city from forming a contiguous bloc, as Ankara vehemently opposes the creation of a Kurdish state.

The YPG are expected to push toward Raqqa but will fall short of entering the mostly Arab city, clearing the way for the Arab section of the SDF doing so.

Another question is the role Turkish troops will play - YPG advances toward Raqqa could be slowed if fighters are forced to remain vigilant while fighting alongside the Turks.

To complicate things further, while Moscow is currently focusing on Aleppo and other regions in support of President Bashar al-Assad, Russian jets are using the same air space as the coalition, prompting fears of a midair collision that could fundamentally alter the calculus.