UNITED NATIONS — A Monday night in March, at 8:30. The sound of helicopters, then a thud, then the smell of bleach, overpowering, followed by a surge of wounded to the local hospital. They were all short of breath. Some vomited. All reeked of bleach.

This is what Dr. Mohamed Tennari, 35, from the Syrian town of Sarmeen, told diplomats on the United Nations Security Council on Thursday in a closed-door meeting called by the United States to draw attention to suspicions of the use of chlorine as a chemical weapon in the war in Syria.

Dr. Tennari showed a video taken at the hospital that he ran. In it, two children are piled on their grandmother’s body. A third, a baby, is on the next bed. Their mouths are open. Gloved hands give them oxygen, then an injection. Dr. Tennari says later that all three children, ages 1 to 3, died. Their parents, too. And their grandmother.

Samantha Power, the American ambassador to the United Nations, told reporters after the session that there were few “dry eyes in the room,” and that the Council would seek to determine who was responsible and hold them accountable.