Brussels (AFP) - As Iraqi forces backed by a US-led coalition inch towards the Islamic State group in Mosul, experts and security sources warn that any assault on the jihadists' main Syrian stronghold of Raqa would be even more difficult.
The US and British defence ministers said on Wednesday they expected an assault to drive IS from its de facto capital of Raqa to begin in the next few weeks.
If Mosul falls, Raqa will be the only major city in either Syria or Iraq under IS control, the vestige of a cross-border "caliphate" the jihadists declared after seizing large parts of both countries in mid-2014.
Tens of thousands of Iraqi fighters are steadily advancing on Mosul backed with air and ground support from a US-led coalition. Washington says a Raqa assault could be run at the same time.
"That has long been our plan and we will be capable of resourcing both," US Defense Secretary Ashton Carter told NBC News before arriving in Brussels Wednesday for a two-day meeting of NATO defence chiefs.
Pleased with progress in the Iraq offensive, military chiefs are talking about overlapping operations in Raqa, though there is still a great deal of caution.
US Lieutenant General Stephen Townsend, commander of the coalition supporting Iraqi forces and Kurdish peshmerga units in the fight against IS, acknowledged Syria was "a very complicated battle space".
"There are a lot of regional security concerns that are in competition there... the Syrian regime's involved, the Russians are involved, Turkey's involved. It's hard," Townsend told reporters Wednesday.
A French source acknowledged that the Syrian side of the fight against IS was "much more complex," saying "obviously not everything is in place to take Raqa tomorrow."
- 'Child's play' -
The Lebanese newspaper L'Orient le Jour put it starkly in a comment piece this week, saying that "compared to the Syrian crisis, the Iraqi problem is practically child's play".
The offensive on Mosul, which began on October 17, was more than a year in the planning, with the coalition, Baghdad and authorities in autonomous Iraqi Kurdistan all taking part.
That operation has its own complications, namely over the role of Iraq's powerful Shiite militias and of Turkey, which considers Sunni-majority Mosul to be part of its natural sphere of influence.
But an operation in Syria - theatre of a proxy battle royale among the great powers - would be even more difficult.
In practical terms there is also the question of who would wage the offensive in a country ravaged by a bloody five-year civil war and broken up by myriad groups all fighting each other.
"There is a difference in nature between Iraq and Syria. In Iraq we are intervening at the invitation of the Baghdad authorities," the French source said.
There would be no such invitation in Syria -- the countries taking part in the anti-IS coalition are opposed to the regime of Syrian President Bashar al-Assad and want to avoid operations that would help him.
The view in Washington is that it must be local fighters that take the lead.
Defense Secretary Carter spoke this week of "capable and motivated local forces that we identify and then enable".
- The Turkish and Kurdish factors -
"At this stage there are only two forces in Syria fighting Daesh, the Syrian Democratic Forces (a Kurdish-Arab coalition backed by Washington) and the rebels of the FSA (Free Syrian Army, backed by Turkey)," the French source said.
Military officials say this is enough, but estimates of available forces range widely from 10,000 to 30,000 men.
Another problem is Turkey's opposition to Kurdish militias, which form a key part of the SDF. Ankara views them as extensions of Turkey's own banned separatist Kurdistan Workers' Party (PKK), making cooperation unlikely.
General Townsend acknowledged the difficulty, saying the US was in talks with Ankara, but clearly stating that in his view the "only force that is capable on any near-term timeline" of beating IS in Raqa is the SDF.
On Wednesday, Washington seemed to be leaning towards Ankara.
"We already are working extensively with the Turkish military in Syria" and this had produced "significant" results, including the seizure of the "very important" town of Dabiq from IS, Carter said in Brussels after meeting his Turkish counterpart.
"So we are looking for other opportunities including further within Syria, to include Raqa. That's been part of our discussions."
On Thursday, Turkish President Recep Tayyip Erdogan said his country's military operation in support of Syrian opposition fighters in northern Syria would extend to Raqa.
Still unclear is how Moscow, fighting to support Assad since last September, will respond.
"Russia is fighting a different war, wiping out the opposition (to Assad) in Aleppo. Raqa clearly doesn't interest them," the French source said.