United Nations Secretary-General Ban Ki-moon has blasted Syria's warring parties, particularly President Bashar al-Assad's government, for committing "atrocious acts" and "unconscionable abuses" against civilians.
Speaking after briefing the UN General Assembly on his 2016 priorities, Mr Ban said harrowing images of starving civilians in the besieged town of Madaya reflected a new low in a war that had already reached "shocking depths of inhumanity."
"Let me be clear: the use of starvation as a weapon of war is a war crime," he told reporters. "I would say they are being held hostage, but it is even worse. Hostages get fed."
The UN Security Council will be briefed tomorrow on the besiegement of some 400,000 people in Syria, diplomats said. The meeting was requested by Britain, France and the United States.
Aid was delivered to Madaya and two Shia villages on Monday for the first time in months. Madaya is besieged by pro-Syrian government forces, while the two villages in Idlib province are encircled by rebels. More aid reached both areas today.
"All sides - including the Syrian government, which has the primary responsibility to protect Syrians - are committing atrocious acts prohibited under international humanitarian law," he said.
He said UN teams in Madaya had seen "scenes that haunt the soul."
"The elderly and children, men and women, who were little more than skin and bones: gaunt, severely malnourished, so weak they could barely walk, and utterly desperate for the slightest morsel," Mr Ban said.
He said some 400 men, women and children in Madaya were in such a dire state from malnutrition and other conditions that they were in danger of dying and needed immediate medical attention, including possible evacuation.
Some 400,000 people in Syria were besieged - about half in areas controlled by the so-called Islamic State group, some 180,000 in areas controlled by the Syrian government and about 12,000 in areas controlled by opposition armed groups, Ban said.
"In 2014, the UN and partners were able to deliver food to about 5% of people in besieged areas. Today, we are reaching less than 1%. This is utterly unconscionable," Mr Ban said.
He urged key regional and world powers, specifically the International Syria Support Group, to press Syria's warring parties for sustained and unimpeded humanitarian access and an end to the use if indiscriminate weapons in civilian areas.
Convoy delivers aid to Madaya
Earlier, a convoy of several trucks carrying aid has entered Madaya for the second time this week.
The Syrian Observatory for Human Rights said that aid trucks have started entering rebel-held town of Madaya near the border with Lebanon and the Shia villages of Kefraya and al-Foua in Idlib province in the northwest of the country.
Syrian state media said six trucks have entered Madaya.
A spokesman for International Committee of the Red Cross said earlier that 44 aid trucks were heading to Madaya, and 21 others to Kefraya and al-Foua.
The trucks were mainly carrying wheat flour and hygiene products.
"According to the ICRC team that entered Madaya, the people were very happy, even crying when they realized that wheat flour is on the way," Dominik Stillhart, ICRC director of operations, told reporters in New York.