Food and medicine have begun to dwindle in the city of Aleppo after an advance by Syrian regime forces effectively cut off the only road into the rebel-held side of the divided city, residents and opposition leaders said Wednesday.

Battles near the Castello road have intensified since last week even as the regime announced a  cease-fire on July 6 that was extended twice. The United Nations said Wednesday it was deeply alarmed by the escalation of violence along the Castello road, particularly because residents in Aleppo’s rebel-controlled neighborhoods are heavily dependent on humanitarian aid.

If regime and allied forces succeed in fully encircling Aleppo’s rebel-held eastern half—with an estimated population of about 300,000—it would become the largest civilian population under siege in Syria.

Food and humanitarian blockades are a common battlefield tactic in Syria’s bloody conflict. The U.N. has listed 19 areas with nearly 600,000 people as under siege. Dozens have died from starvation and untreated medical conditions in besieged areas, according to local activist groups.

There are only enough food supplies for 145,000 people for up to one month in the rebel side of Aleppo, according to the U.N.

Osama Abu al-Izz, a doctor with the Syrian American Medical Society in Aleppo, said some medical supplies could last for up to three months but that some medicine—especially for emergency care—was already beginning to run out because of more than a month of intense regime air attacks that wounded many.

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