BEIRUT • With Russian warplanes in the air and Iranian special forces on the ground, an emboldened Syrian President Bashar al-Assad is turning back to the biggest trophy in his country's civil war, and, this time, Europe is also on the front line.

As his troops head towards Aleppo, Syria's former commercial hub and largest city, helicopters are dropping warnings to residents to vacate areas. Should Aleppo and other Sunni towns fall to Mr Assad's forces, the potential for another, larger, wave of refugees would be nightmarish, according to one official in a European government.

Already, at least 70,000 Syrians have fled a government offensive on rebel-held areas south of the city of Aleppo in the past three days, reported the BBC, citing an activist.

The province is roughly divided between Mr Assad and the militants. If they see government forces advancing, Syrians "who have been on the fence will be convinced that it's time to go", said Mr Ghanem Nuseibeh, founder of London-based consulting firm Cornerstone Global Associates.

"You would expect another wave of refugees to try to head towards Turkey and eventually to Europe before the regime closes the border," he said.

More than four years of fighting has displaced 71/2 million people within Syria and turned it into a country that exists only in name.

For the United States, the question is how much political and military capital it wants to pour into another Middle Eastern country, while rivals Russia and Iran take advantage of its reticence to prop up their ally in the region.

Europe, meanwhile, is faced with one of the biggest geopolitical challenges in decades as hundreds of thousands of Syrians seek a haven.

The number of migrants who have arrived in the European Union in the first nine months of this year is already more than twice the total during the whole of last year, and another two million are still in Turkey.

US Secretary of State John Kerry said yesterday that the US is working to avoid the "total destruction" of Syria, and plans a meeting in the coming days with Russian, Saudi and Turkish leaders.

Washington considers that it bears the responsibility "to try and avoid the complete and total destruction of Syria", fearing the potential fallout across the region and a possible surge in migration.

Mr Kerry also said he feared the consequences of Russia's air strikes in Syria. "Our fear (is that)... Russia is simply there to prop up Assad," he said, warning that Moscow's air campaign might "attract more jihadists to the fight".

BLOOMBERG, AGENCE FRANCE-PRESSE