UNITED NATIONS — The head of the United Nations said on Thursday that the warring parties in Syria were committing war crimes by withholding food from civilians. He called for all sides to lift their sieges immediately and unconditionally “as a confidence-building gesture” ahead of peace talks scheduled for Jan. 25.

The statement came as aid convoys delivered food and medicine for the second time this week to three besieged Syrian towns. United Nations officials have said that it took more than three months to secure access to the towns, and that many residents, including children, had died of hunger and disease in that time.

“Let me be clear: The use of food as a weapon of war is a war crime,” Secretary General Ban Ki-moon said on Thursday in a speech to the General Assembly. “All sides, including the Syrian government, which has the primary responsibility to protect Syrians, are committing atrocious acts prohibited under international humanitarian law.”

The remarks were among the toughest yet on the subject by Mr. Ban, who is in the final year of his 10-year term. The United Nations estimates that about 400,000 Syrians have been denied humanitarian aid because their towns or neighborhoods are besieged by one side or another in the civil war.