Syrian opposition activists say government forces and their allies have closed the only road leading into and out of rebel-held parts of the northern city of Aleppo, besieging hundreds of thousands of people.

The Britain-based Syrian Observatory for Human Rights says government forces and members of Lebanon's Hezbollah group reached the Castello road Sunday, closing it and raising fears of a humanitarian crisis.

Government troops reached a point where they could fire on the road earlier this month, effectively cutting it off, though some people still made the dangerous journey.

Aleppo-based activist Bahaa al-Halaby told The Associated Press that rebel-held neighborhoods in the city are under "full siege."

The U.N. has warned that nearly 300,000 people rely on the Castello road for travel, food and medicine.