Abu Khaled helps his children as he demonstrates how to get into a makeshift shelter near Damascus. Khaled, who works as a farmer, dug an underground shelter for his family to hide in during shelling from forces loyal to President Bashar al-Assad. (Yousef Albostany/Reuters)

With more than 5 million people internally displaced, a suspected polio outbreak, and starvation threatening, the United Nations and aid agencies say that just a trickle of the required assistance is getting into war-ravaged Syria as the harsh winter months loom.

After more than 21 / 2 years of conflict, the accounts of struggling civilians paint a portrait of abject human suffering amid what the World Health Organization has deemed the world’s worst humanitarian crisis.

The U.N. humanitarian chief, Valerie Amos, on Friday chastised both sides for failing to improve access along the lines laid out in an Oct. 2 U.N. statement and described the situation as a “race against time.”

The crisis has intensified over the past month, with the WHO reporting 22 suspected cases of polio in Deir al-Zour — which, if confirmed, would mark the first outbreak of the debilitating virus in Syria in 14 years.

U.N. agencies are mobilizing an emergency plan to vaccinate more than 2.5 million children against polio, but with cross-border operations from neighboring countries and operations across front lines hampered by red tape and security concerns, they concede it will be a major challenge.

In addition to polio, communicable diseases such as measles and hepatitis are threatening Syria’s vulnerable.

Some of the most desperate conditions are in rebel-held enclaves under siege by government forces, such as those on the outskirts of the capital, Damascus, and in Homs and Aleppo. There, field hospital doctors say, they have documented children dying of starvation.

“My children dream of bread,” said Zahra, a 26-year-old mother from the Damascus suburb of Moadamiya who is pregnant with her fourth child and did not wish to give her last name. The area has been under siege for almost a year, and Zahra said her family is being slowly starved but can’t leave because their way is blocked by snipers and checkpoints.

Two of her daughters have anemia because of iron deficiency. Family meals have been reduced to one a day, normally a broth made from whatever can be scraped together.

“What keeps us going is the hope that things will get better, that we will get out,” she said. She tried to leave three times in an evacuation effort brokered by a pro-government nun this month. But the effort was aborted when shells hit just yards from the area where hundreds of civilians were waiting to board buses to leave.

The United States has accused the Syrian government of deliberately preventing the delivery of lifesaving relief, describing it as “unconscionable.”

Rasheed al-Wazzan, a physician in Moadamiya’s field hospital, said six people, including four children, have died of malnutrition in the suburb.

Although more than 100,000 people have been killed in the war, according to U.N. and activist estimates, there are no figures on how many thousands more may have died from treatable conditions due to a lack of supplies and Syria’s devastated health-care system.

Doctors Without Borders, which says it has confirmed cases of malnutrition, expects cases to spread in northern Syria during the winter months as food supplies run low and inflated prices mean that many struggle to feed their families.

The aid group is translating malnutrition guidelines that it uses in Africa into Arabic to distribute to Syrian doctors.

Malnutrition also threatens the old city of Homs, in central Syria, where residents are also penned in by government troops. Supplies gained from scavenging in abandoned houses are drying up, residents say.

Western diplomats say that improving humanitarian relief access, also a key demand of the Syrian opposition ahead of a planned peace conference in Geneva this month, is now their key focus.

Given the West’s reservations over the opposition’s other request — weapons to change the balance of power on the ground — diplomats are hoping that movement on access will encourage the opposition’s participation in peace talks.

“We want to see an improvement on humanitarian access before Geneva as a confidence-building measure,” said one Western diplomat, who spoke on the condition of anonymity to discuss the matter more openly. He said securing it hinges on efforts by the Russians to pressure the Syrian government.

Aid agencies say cross-border aid efforts from neighboring countries such as Jordan and Lebanon and cross-line operations within Syria must improve.

“We’ve seen the international community has been able to deal on chemical weapons. We think it's incredibly cynical not to also do it with lifesaving humanitarian access,” said Christopher Stokes, a Brussels-based general director for Doctors Without Borders. “The core of the problem is the government, but insecurity doesn’t help.”

Western sanctions are also having an impact, combining with the destruction of factories in the war to devastate Syria’s pharmaceutical industry. Some medicines now cost 30 times their price before the uprising, according to Tarik Jasarevic, a WHO spokesman.

“There are diseases spreading, like scarlet fever, that we haven’t seen for years,” Majed Abu Ali, a spokesman for the medical office for Eastern Ghouta, a swath of countryside east of Damascus, much of which has been under siege for months. He said cases of leishmaniasis, a skin disease dubbed the “Aleppo boil” because of its prevalence in the northern city, has begun to appear.

“We can’t get vaccines for anything,” he said. “Then there's the water. We can’t be sure it’s clean, and there’s no gas to boil it.”

More than 2 million Syrians, 10 percent of the population, have fled the country. Stokes, of Doctors Without Borders, said he expects a further exodus as winter sets in if access doesn’t improve.

“It’s 2013, in a world where technology and development is at its peak,” said Walid Fares, an activist from the Old City in Homs. “People around the world think they are in an advanced stage of thinking and good living. Homs is somewhere else on this planet — everyone knows its address, yet nothing is done.”