One aid worker — who, in a sign of the political challenges of delivering aid in Syria, asked that his organization not be identified — said he recently met Syrian health workers who reported a dozen cases of apparent malnutrition in a government-held Damascus suburb. He suspected that the situation could be far worse in rebel-held areas.

Lack of medical care and clean water exacerbates the problem. So does the fact that Syrians have little experience diagnosing or treating malnutrition. Particularly troubling, aid workers say, are reports of mothers who stop breast feeding, unaware that it is the best way for even a malnourished mother to keep her child alive.

Some aid groups are trying to train Syrian doctors to use simple tools that measure upper arm circumference to assess malnutrition, as convincing data on its prevalence could help spur a stronger international response. Aid workers caution against overblown claims that could discredit such efforts. Some government supporters even dismissed the images of bone-thin children from blockaded areas as propaganda after several thousand civilians were evacuated from the encircled Damascus suburb of Moadhamiya in recent weeks, looking exhausted, shellshocked and thin, but not on the verge of starving to death.

But an entire population does not have to appear skeletal for malnutrition deaths to be real, the experts say. Malnutrition, they say, strikes the most vulnerable first: babies and children; those suffering from diarrheal diseases; those who need extra nutrition to recover from wounds or manage chronic illnesses; and those who lack the money or connections to obtain the food they need.

In traumatic situations, cases may go unnoticed until they are advanced, when victims reach “a point of no return,” said Dr. Vincent Iacopino, a senior medical adviser to Physicians for Human Rights. Unable to absorb calories, many do not recover without sophisticated medical care, even if given the food portions of others, he said.

Regardless, aid workers say, the fact that military blockades are preventing people in such acute need from receiving aid is in itself a human-rights violation. It matters little, they say, whether those suffering are technically the first victims of incipient famine, something no organization has the access or data to determine, or simply sick people who need treatment.

“It shouldn’t have to take people starving to support these people,” said another aid worker.

The very unlikelihood of hunger in Syria galls those suffering from it. “It’s very strange to know that the food is only five minutes away from you,” said Qusai Zakarya, a spokesman for a rebel council in Moadhamiya, who said he recently spoke on the phone to a friend who was eating a cheeseburger in the wealthy neighborhood of Mezze just a few miles away.