The U.N. humanitarian chief warned Thursday that despite increased aid deliveries in Syria, people are still starving without medical care and said the world should be "ashamed" at the devastating human toll in five years of "senseless" fighting.

Stephen O'Brien demanded unimpeded humanitarian access and told the U.N. Security Council that it must not squander the opportunity presented by peace talks in Geneva, which wrapped up their latest round on Wednesday with virtually no progress.

"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."

O'Brien decried the Syrian government's removal of medicine and medical supplies from aid convoys, calling the practice "inhumane" and a violation of international humanitarian law.

And in a departure from his prepared text, he warned 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 behavior."

On a positive note, O'Brien said the World Food Program reached 3.7 million people with food and the World Health Organization vaccinated over 2.1 million children against polio in March.

The number of humanitarian convoys crossing borders and crossing combat lines has increased, with assistance provided to over 778,000 people in besieged, hard-to-reach and other priority areas since January, he said, and 14 high-altitude airdrops since April 10 are helping some 100,000 people in Deir el-Zour.

Nonetheless, O'Brien said, "current levels of access still leave civilians starving and without medical care."

He pointed to the first U.N. visit to Daraya since 2012 on April 16, which found 80 to 90 percent 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."

O'Brien said a U.N. request to send aid to the town has not yet been granted. He said the U.N. has submitted requests to send aid convoys to 35 towns in critical need in May, including Daraya and Douma, and he urged immediate approval.