The Islamic State group and other jihadists on Monday cut a vital supply route linking the west of Syria's second city Aleppo with other government-held territory, a monitoring group said.
The road between Aleppo and the town of Khanasser to the southeast was the only way regime forces and civilians living in government-controlled neighbourhoods of the city could travel to surrounding provinces.
If government forces are unable to recapture the road, it could slow an offensive they launched in the countryside around Aleppo earlier this year.
And it could worsen severe shortages of food and water for civilians.
"Jihadists from the Caucasus and from (China's mainly Muslim region of) Xinjiang, as well as the jihadist group Jund al-Aqsa, cut the route from the south after a surprise attack," said Syrian Observatory for Human Rights chief Rami Abdel Rahman.
"And fighters from IS cut off a different part of the route from the northern side at the same time," Abdel Rahman said.
Thousands of foreign fighters have flocked to Syria over the past two years, many of them taking up arms with Al-Qaeda affiliate Al-Nusra Front, IS or Jund al-Aqsa.
It is not the first time that the government's supply route to Aleppo has been cut.
Rebel cut it in 2013 and IS did so last year before being pushed back.
The new setback comes with government troops on the offensive north and west of Aleppo, where rebel forces in the east of the city are almost completely surrounded.