The United States has demanded that Syria let humanitarian supplies into rebel-held suburbs of Damascus under siege by the army for months.
Washington said civilians are in desperate need of food, water and medicine.
It cited reports of children dying of malnutrition a few kilometres from President Bashar al-Assad's palace.
Footage obtained by the BBC from the suburb of Yarmouk showed families struggling to find enough to eat.
The Syrian army said the rebel-held areas must surrender or starve.
At least three of Damascus' suburbs - Yarmouk, Eastern Ghouta and Moudamiyah - have been besieged by government forces for several months.
The situation has become so desperate that earlier this week Muslim clerics issued a religious ruling allowing people to eat cats, dogs and donkeys just to survive.
Those animals are usually considered unfit for human consumption in Islam.
In a statement on Friday, US State Department spokeswoman Jen Psaki said: "We call on the Syrian regime to immediately approve relief convoys."
She added that in Moudamiyah "people have been without basic necessities for nearly a year, and the regime's deliberate prevention of the delivery of life-saving humanitarian supplies to thousands of civilians is unconscionable".
Syrian activists say they are now starting to record the first deaths of complications caused by malnutrition.