Riyadh (AFP) - With rebel forces facing the prospect of a crushing defeat by Syria's Russian-backed regime, their allies Saudi Arabia and Turkey may send in limited numbers of ground troops, analysts say.

Riyadh has left open the possibility of deploying soldiers, saying it would "contribute positively" if the US-led coalition against the Islamic State (IS) jihadist group in Syria decides on ground action.

The fate of Saudi-backed Syrian armed opposition groups fighting to topple President Bashar al-Assad is also a major concern for Riyadh.

"I think Saudi Arabia is desperate to do something in Syria," said Andreas Krieg of the Department of Defence Studies at King's College London.

Krieg said the "moderate" opposition is in danger of being routed if Aleppo falls to the regime, whose forces have closed in on Syria's second city, backed by intense Russian air strikes.

"This is a problem for Saudi and Qatar as they have massively invested into Syria via the moderate opposition as their surrogate on the ground," said Krieg, who is also a consultant to Qatar's armed forces.

Russia, which along with Saudi Arabia's regional rival Iran is a major Assad ally, has meanwhile accused Turkey of "preparations for an armed invasion" of Syria.

Turkish President Recep Tayyip Erdogan called the claims "laughable".

On Saturday, Damascus issued a grim warning to both Riyadh and Ankara against any military intervention on the ground.

"Let no one think they can attack Syria or violate its sovereignty because I assure you any aggressor will return to their country in a wooden coffin, whether they be Saudis or Turks," Foreign Minister Walid Muallem said.

- Iran warning -

The commander of Iran's elite Revolutionary Guard mockingly said Saudi Arabia wouldn't dare send ground troops, and that any such intervention would be suicidal.

"I don't think they would dare do that... If they do, they will inflict a coup de grace on themselves," Major General Ali Jafari said.

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Aleppo province is among the main strongholds of Syria's armed opposition, which is facing possibly its worst moment since the beginning of the nearly five-year war, at a time when peace efforts have stalled.

The Saudi-backed opposition umbrella group, the High Negotiations Committee, says it will not return to peace talks which recently collapsed in Geneva unless its humanitarian demands are met.

"The Saudis believe that the chance of a peaceful solution for the Syrian crisis is very limited," said Mustafa Alani of the independent Gulf Research Centre.

"They don't see that there is a real pressure on the regime to give major concessions... They think eventually it will have to end in the battlefield.

"Turkey is enthusiastic about this option (of ground troops) since the Russians started their air operation and tried to push Turkey outside the equation," Alani added.

He said the Saudis are serious about committing troops "as part of a coalition, especially if the Turkish forces are going to be involved".

But he and other analysts said Saudi involvement would be limited, given its leadership of a separate Arab coalition fighting in Yemen for almost a year and guarding the kingdom's southern border from attacks by Iran-backed Yemeni rebels.

- Saudis 'overstretched' -

"They are overstretched. But in principle I think they will not hesitate to send a certain number of their fighters to fight in Syria," Alani said, adding that this would probably include Saudi special forces.

Turkey and Saudi already belong to a US-led coalition which officially has 65 members. It has been bombing IS in Syria and Iraq, as well as training local forces to fight the extremists.

Krieg said that with Saudi and other Gulf kingdoms "bogged down" in Yemen, he could only foresee a possible expansion of "train and equip" missions involving Gulf special forces to help rebels in Syria.

"Saudi and Qatar have already networks on the ground," he said, viewing Doha as a link between Riyadh and Ankara as relations improve.

On Friday, US Central Command spokesman Pat Ryder welcomed Saudi Arabia's willingness to send soldiers against IS.

The United States has been calling on coalition members to do more.

In November, the United Arab Emirates said it was also ready to commit ground troops against jihadists in Syria.

Jane Kinninmont, senior research fellow at London's Chatham House, said Riyadh is more interested in the Yemen war than battling IS.

"But what you might see is small numbers of ground troops and perhaps also special forces which would be there partly to make a symbolic point that Saudi Arabia is supporting the fight against ISIS," she said, using another acronym for IS.

She declared herself "a bit sceptical" about potential Turkish army involvement in Syria, "but we might see them having some kind of interest in containing Kurdish influence".