"Let me be clear: the use of starvation as a weapon of war is a war crime," Ban told reporters.
"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," he said.
Ban spoke after a second convoy carrying food and other necessities entered Madaya yesterday where residents told AFP they had been surviving on soup from boiled grass.
France, Britain and the United States called for an emergency meeting of the Security Council to press demands for an end to sieges ahead of peace talks planned for January 25 in Geneva.
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French Ambassador Francois Delattre told AFP that the meeting, expected to be held tomorrow, "will draw the world's attention to the humanitarian tragedy that is unfolding in Madaya and in other towns in Syria."
The United Nations is struggling to deliver aid to about 4.5 million Syrians who live in hard-to-reach areas, including nearly 400,000 people in besieged areas.
"These children and women and men are struggling to survive without food or medicine," he said.
Humanitarian aid access is seen as a key confidence-building measure ahead of new round of Syrian peace talks.
Diplomats suggested that the talks would have no chance of success if the humanitarian crisis remains desperate.
"It will be difficult for them to negotiate while their children and close ones are threatened with famine or death," said a Security Council diplomat.
The UN envoy for Syria, Staffan de Mistura, said yesterday that the permanent Security Council members -- Britain, China, France, Russia and the United States -- had pledged to take "immediate action" to push for deliveries of aid to besieged areas.
UN High Commissioner for Human Rights Zeid Ra'ad Al Hussein, speaking in Qatar, said that those responsible for the starvation sieges should face justice.