The UN says the world should be "ashamed" after warning people in Syria are starving without medical care.
Stephen O'Brien, the UN's under secretary-general for humanitarian affairs and emergency relief, said that, despite increased aid deliveries in Syria, people are still starving.
Mr O'Brien demanded unimpeded humanitarian access and told the UN Security Council that it must not squander the opportunity presented by peace talks in Geneva.
"The people of Syria cannot afford to see the situation move backward again," he warned. "If the international community fails to maintain momentum politically, with the cessation of hostilities, and with humanitarian access, the situation will only spiral further out of control."
Mr O'Brien criticised the Syrian government's removal of medicine and medical supplies from aid convoys, calling the practice "inhumane" and a violation of international humanitarian law.
He said: "Deliberately deprived of food and medicine, many face the most appalling conditions of desolation, hunger and starvation. We must all be ashamed that this is happening on our watch."
And in a departure from his prepared text, he warned Syrian President Bashar Assad's government that those responsible for the "unnecessary suffering and loss of life" will be held accountable when the fighting stops, saying "there can never be impunity for this behaviour".
Mr O'Brien said the World Food Programme had reached 3.7 million people with food and the World Health Organisation vaccinated over 2.1 million children against polio in March.
The number of humanitarian convoys crossing borders and combat lines has increased, with assistance provided to more than 778,000 people in besieged, hard-to-reach and other priority areas since January, he said.
He added that 14 high-altitude air drops since April 10 are helping around 100,000 people in Deir el-Zour.
But Mr O'Brien continued: "Current levels of access still leave civilians starving and without medical care."
He pointed to the first UN visit to Daraya since 2012 on April 16, which found 80 to 90% of the besieged town destroyed, with electricity, water and sanitation facilities beyond repair, and households eating one meal a day with the poorest "sending children into the streets to beg and being reduced to eating grass and wild vegetation".
Mr O'Brien said a UN request to send aid to the town has not yet been granted. He said the UN has submitted requests to send aid convoys to 35 towns in critical need in May, including Daraya and Douma, and urged immediate approval.