The suggestion by Saudi Arabia that it send ground troops to Syria might mean the limited deployment of Saudi special forces alongside their US counterpart or a more ambitious intervention, probably in combination with the Turkish army.

It might also be a desperate last throw of the dice to rescue a bankrupt policy as it becomes clear that a five-year effort by Saudi Arabia, Turkey, Qatar and the Gulf monarchies to overthrow Syria’s President, Bashar al-Assad, has failed. The Syrian army is advancing on all fronts, backed by Russian air strikes, and has cut off opposition forces in Aleppo.

Saudi policy has become more aggressive and proactive over the past year as it has come under the control of the deputy Crown Prince and Defence Minister, Prince Mohammed bin Salman, who is seen as the power behind the Saudi throne. But much remains unclear about any proposed action by Saudi ground troops in the Syrian civil war, though the US has welcomed the idea of extra help from Saudi Arabia and anybody else who wants to join the fight against Isis.

Syrians Flee Aleppo to Huge Refugee Camp near Turkish Border

If Saudi Arabia becomes involved in any more ambitious venture in Syria it is likely to be in association with Turkey; but Turkey’s policy towards sending its army across the border is ambivalent because to do so would be highly risky.

President Recep Tayyip Erdogan is appalled by the prospect that the war in Syria may conclude with Mr Assad still in power and the de facto Syrian Kurdish state of Rojava controlling half of the 550-mile long Syrian-Turkish border. Turkey particularly wants to prevent the Syrian Kurdish militia force, the People’s Protection Units (YPG), advancing west of the Euphrates and linking up with the Kurdish enclave of Afrin. This would cut the supply lines of Isis and the non-Isis opposition from Turkey to Aleppo.

In pictures: Russian air strikes in Syria Show all 19 1 /19 In pictures: Russian air strikes in Syria In pictures: Russian air strikes in Syria Syrian boys cry following Russian air strikes on the rebel-held Fardous neighbourhood of the northern embattled Syrian city of Aleppo Getty In pictures: Russian air strikes in Syria Russian defense ministry spokesman Major General Igor Konashenkov speaks to the media in Moscow, Russia. Konashenkov strongly warned the United States against striking Syrian government forces and issued a thinly-veiled threat to use Russian air defense assets to protect them AP In pictures: Russian air strikes in Syria Syrians wait to receive treatment at a hospital following Russian air strikes on the rebel-held Fardous neighbourhood of the northern embattled Syrian city of Alepp Getty In pictures: Russian air strikes in Syria Russian Deputy Defense Minister Anatoly Antonov speaks at a briefing in the Defense Ministry in Moscow, Russia. Antonov said the Russian air strikes in Syria have killed about 35,000 militants, including about 2,700 residents of Russia AP In pictures: Russian air strikes in Syria Jameel Mustafa Habboush, receives oxygen from civil defence volunteers, known as the white helmets, as they rescue him from under the rubble of a building following Russian air strikes on the rebel-held Fardous neighbourhood of the northern embattled Syrian city of Aleppo Getty In pictures: Russian air strikes in Syria Civil defence members rest amidst rubble in a site hit by what activists said were airstrikes carried out by the Russian air force in the town of Douma, eastern Ghouta in Damascus, Syria Reuters In pictures: Russian air strikes in Syria A girl carrying a baby inspects damage in a site hit by what activists said were airstrikes carried out by the Russian air force in the town of Douma, eastern Ghouta in Damascus, Syria Reuters In pictures: Russian air strikes in Syria Civilians and civil defence members look for survivors at a site damaged after Russian air strikes on the Syrian rebel-held city of Idlib, Syria Reuters In pictures: Russian air strikes in Syria Civilians and civil defence members carry an injured woman on a stretcher at a site damaged after Russian air strikes on the Syrian rebel-held city of Idlib, Syria Reuters In pictures: Russian air strikes in Syria Volunteers from Syria Civil Defence, also known as the White Helmets, help civilians after Russia carried out its first airstrikes in Syria In pictures: Russian air strikes in Syria The aftermath of Russian airstrike in Talbiseh, Syria In pictures: Russian air strikes in Syria Smoke billows from buildings in Talbiseh, in Homs province, western Syria, after airstrikes by Russian warplanes AP In pictures: Russian air strikes in Syria Russian Air Forces carry out an air strike in the ISIS controlled Al-Raqqah Governorate. Russia's KAB-500s bombs completely destroy the Liwa al-Haqq command unit In pictures: Russian air strikes in Syria Caspian Flotilla of the Russian Navy firing Kalibr cruise missiles against remote Isis targets in Syria Â© TASS/ITAR-TASS Photo/Corbis In pictures: Russian air strikes in Syria Russia claimed it hit eight Isis targets, including a "terrorist HQ and co-ordination centre" that was completely destroyed In pictures: Russian air strikes in Syria A video grab taken from the footage made available on the Russian Defence Ministry's official website, purporting to show an airstrike in Syria In pictures: Russian air strikes in Syria A release from the Russian defence ministry purportedly showing targets in Syria being hit In pictures: Russian air strikes in Syria Russia launched air strikes in war-torn Syria, its first military engagement outside the former Soviet Union since the occupation of Afghanistan in 1979. Russian warplanes carried out strikes in three Syrian provinces along with regime aircraft as Putin seeks to steal US President Barack Obama's thunder by pushing a rival plan to defeat Isis militants in Syria In pictures: Russian air strikes in Syria Caspian Flotilla of the Russian Navy firing Kalibr cruise missiles against remote Isis targets in Syria, a thousand kilometres away. The targets include ammunition factories, ammunition and fuel depots, command centres, and training camps Â© TASS/ITAR-TASS Photo/Corbis

Any move across the border into Syria by Turkey and allies such as Saudi Arabia has become more complicated, and potentially more costly, since the Russian military intervention on 30 September last year and the shooting down of a Russian bomber by a Turkish F-16 fighter on 24 November. A cross-border move might now provoke the use of Russian aircraft and anti-aircraft missile systems against the Turks and anybody else involved.

Armed intervention by the Sunni states could be presented as the creation of “a safe zone” for the tens of thousands of displaced people in the area, though it certainly would not be safe as it would be in the centre of a battle zone. There is the danger that any Saudi-Turkish ground assault would be in association with local opposition fighters grouped under the umbrella organisation Jaish al-Fatah; this is supported by Turkey and Saudi Arabia and is led by the al-Nusra Front, al-Qaeda’s Syrian branch, and by the ideologically similar Ahrar al-Sham.