He spoke after U.N. convoys arrived in Syrian towns to deliver food to malnourished residents.
"U.N. teams have witnessed scenes that haunt the soul," Ban said. "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."
Ban said the United Nations and partners delivered food to about 5% of people in areas struck by civil war in 2014, compared to 1% Thursday. That situation is "utterly unconscionable," he said.
"Let me be clear: the use of starvation as a weapon of war is a war crime," he said. "All sides -- including the Syrian government, which has the primary responsibility to protect Syrians -- are committing this and other atrocious acts prohibited under international humanitarian law."
Aid convoys arrive
The second wave of aid convoys entered the besieged Syrian cities of Madaya
, al-Fouaa and Kefraya on Thursday evening, delivering desperately needed food and humanitarian supplies to residents, the International Committee of the Red Cross said.
Pawel Krzysiek, spokesman for ICRC Syria, tweeted that all trucks in the convoys had entered the blockaded cities, and offloading of their cargo had begun.
"We now meet the families to talk about their needs," he said on Twitter.
Madaya's 40,000 residents have been living under siege by Syrian government forces and allied militias for months, according to U.N. officials. Before an earlier convoy of aid arrived Monday, bringing many starving residents to tears
, Madaya had received no foreign aid since October.
In al-Fouaa and Kefraya, two towns in the country's northwest, about 20,000 have been suffering under a rebel blockade, said Dibeh Fakhr, the ICRC's Near and Middle East spokeswoman.
The relief mission is being conducted as part of a U.N.-brokered deal, according to which aid must be delivered in both regions simultaneously, Fakhr said.
Earlier Thursday, the Madaya-bound convoy of 44 trucks arrived on the outskirts of the city, in a mountainous area 25 kilometers (15 miles) northwest of Damascus, awaiting word that the other convoy of about 20 trucks had reached al-Fouaa and Kefraya from Damascus, she said.
No plans to evacuate the starving
A U.N. source traveling in Thursday's convoy to Madaya said there were no plans to evacuate from that city the 400 residents whom U.N. officials had said needed treatment urgently. Instead, doctors and nutritionists with the convoy will treat those cases, the source said.
The civil war
in Syria is nearing its five-year mark, with the brutal regime of President Bashar al-Assad pitted against rebels. The terror group ISIS has gained large swaths of the country and neighboring Iraq.
More than 250,000 Syrians -- mostly civilians -- have been killed, according to the United Nations. About 10.5 million Syrians have fled their homes -- and more than 4 million of those have left the country, playing a large part in Europe's migrant crisis.