At least 23 reported dead and hundreds malnourished in the besieged town of Madaya outside Damascus.
As winter takes its toll on Syria, at least 400,000 people are cut off from food and medical supplies.
The government and opposition are laying siege to towns and villages, according to the United Nations.
Roughly 227,000 people are trapped by Syrian government forces in three key areas.
Eastern Ghouta, outside Damascus, is a rebel stronghold. About 176,000 people are said to be cut off there.
About 9,000 people are trapped in the Damascus suburb of Darayya and in the western town of Zabadani.
Madaya, to the south, has been under siege by allies of Syrian President Bashar al-Assad’s government since July. The UN says 42,000 people are stuck there.
Abu Khalil is one of those trapped in Madaya. He says the world has been ignoring the plight of those affected.
“The situation is becoming so difficult,” he told Al Jazeera.
“At least 26 more people have died from starvation and 50 others are now unconscious. We don’t understand how the world could do nothing to resolve this crisis after witnessing such tragedy.
“Civilians, including women and children, are dying because of the use of this cowardly weapon.”