While the stark differences between the American and Russian positions were outlined in civil tones, that diplomatic restraint was abandoned when Walid al-Moallem, the Syrian foreign minister who will lead the Syrian government’s face-to-face talks with the opposition, took the floor and accused Arab nations of financing terrorism and conspiring to destroy his country.

Speaking for more than 30 minutes, Mr. Moallem also accused insurgents of “sexual jihad” by using brainwashed women as sex slaves and engaging in incest. When Mr. Ban asked that Mr. Moallem wind up his lengthy speech, the Syrian official shot back: “You live in New York, I live in Syria.”

After Mr. Ban again urged him to be concise, Mr. Moallem said he would conclude soon, adding that “Syria always keeps its promises.” But he continued with his denunciations of the opposition and Mr. Ban later lamented that his injunction that participants take a constructive approach to the crisis “had been broken.”

Ahmad Assi al-Jarba, the president of the Syrian opposition, opened with the story of Hajar al-Khatib, 11, who he said was shot by government forces as she rode a bus to school in Rastan near the city of Homs in May 2011. That was in the early days of the protest movement that set off the uprising. “Ten thousand children have died because of the Syrian Army,” he added, singling out not only Mr. Assad but the army, which many pro-government Syrians distinguish from the political leaders as an object of patriotism.

Mr. Jarba has said from the start that the Syrian opposition will never accept a role for Mr. Assad in a transitional government and he wondered aloud if the negotiators that the Syrian president had sent to Switzerland were prepared to contemplate that outcome. “We want to be sure we have a Syrian partner in this room,” Mr. Jarba said, adding, “Do we have such a partner?”

Asked whether the United States had any way of putting more leverage on the Assad government, Mr. Kerry suggested that the Obama administration would support “augmented” support for the opposition, among other options. But Mr. Kerry was vague about those options, and the White House has been extremely reluctant to use force in Syria or to even treat the Syria crisis as its principal foreign policy challenge.