The opposition-held sector of Syria's divided city of Aleppo has been cut off from the outside world in recent days by an escalation of air and artillery strikes on the only road in, putting hundreds of thousands of people under effective siege.
A government campaign to fully capture Aleppo would most likely bury what little hope remains of reviving a diplomatic effort to end the five-year-old civil war, after talks and a ceasefire sponsored by the United States and Russia fell apart earlier this year.
Aleppo, Syria's largest city before the war when it had more than 2 million people, has been divided for years into rebel and government sectors, and capturing all of it has been one of President Bashar al-Assad's biggest goals.
An estimated 350,000 people are still thought to live in the rebel sector, in harsh conditions made worse by the latest attempt to besiege them by cutting off the last remaining route out, the Castello Road, named for Aleppo's old castle.
"The regime was not able to cut the road by land, so it has decided to keep the planes in the sky continuously, hitting everything that passes, regardless of what it is," said Zakaria Malahifji, senior official in the Aleppo rebel group Fastaqim.
"Whoever wants to go on the Castello road is undertaking a suicide mission," he said. "It's been this way for 10 or 12 days. The situation was difficult before - it was targeted and people were crossing with difficulty - but now it is almost cut, nobody dares to use it," he said, speaking at his group's office in the Turkish city of Gaziantep near the Syrian border.
The international focus in Syria in recent weeks has partly turned toward the conflict with Islamic State fighters, as both the government and its enemies have made gains at the expense of the ultra-hardline Islamist militants on several fronts.
But the separate hope of foreign powers -- that the wider civil war could also be resolved -- has broken down, with Aleppo potentially the biggest battlefield of all. Hundreds of people have been killed there since peace talks broke off.
Assad vowed in a speech last week to recapture "every inch" of Syria. Aleppo, he said, would be a "graveyard" for the ambitions of his regional foe, Turkish President Tayyip Erdogan, who has backed rebel groups.
A pro-Damascus source, who spoke on condition of anonymity, said the reference was a signal of his intentions: "Why this threat? Because there are preparations for something big in Aleppo."