The UN World Food Programme on Friday said some Syrians in the besieged areas of Daraya and Deir al Zor have been forced to eat grass because food supplies are cut off.
"In the most severe cases, they are enduring entire days without eating, sending children to beg and eating grass/wild vegetation," a report said.
Deir al Zor is under siege by DAESH terrorists, while Daraya is besieged by regime forces and has become a focus of UN efforts to get aid to all of Syria.
Families in the two cities were unable to eat more than one meal per day and giving priority to children, said the WFP report, a survey of Syrian food market conditions in February.
Fresh bread was "sporadically available at an extortionate cost" in Daraya, 30 times above the market price in nearby Damascus, as price of rice was 17 times higher.
Despite a widespread cessation of hostilities deal that has lasted almost three weeks, the Bashar al-Assad regime is still refusing aid and blocking medical care for those in need in six besieged towns, according to the United Nations.
On Thursday, UN humanitarian advisor Jan Egeland said countries backing the Syrian peace talks had given the Syrian regime seven days to respond to a UN request to deliver aid.
"It is in violation of international law to prevent us from going," he said.
"In Daraya there has been fighting, but we had a very clear impression that we will not be having any problems in delivering if we get the two sides to agree to the cessation of hostilities so that we can deliver to the few thousand people there, civilians who are in a very, very difficult position," Egeland added.
The UN also tried to air drop food, but failed at its first attempt because the plane had to fly high and fast to avoid the threat of surface-to-air missiles, causing the parachutes to fail due to the severe jolt when they opened.
On Wednesday, as Russia continued to withdraw its military forces from the country, 102 humanitarian organisations signed a joint statement to mark the fifth anniversary of the conflict's beginning and demanded unconditional access to all communities in Syria.
More than 11 million Syrians from a population of nearly 23 million have been forced to leave their homes during the five-year-old conflict, including 4.8 million who have fled the country.