DURBAN, South Africa — Facing sharp criticism from fellow envoys, environmental activists and one impassioned heckler, the chief American negotiator at a climate conference here on Thursday shifted his position — or at least his language — on a timetable for a new set of international talks.

Todd D. Stern, the Obama administration’s special envoy for climate change, was put on the defensive by a narrative developing here that the United States opposed any further action to address global climate disruption until after 2020, when the 1997 Kyoto Protocol, a primary United Nations climate agreement, and voluntary programs negotiated more recently will have run their course.

He firmly denied that the United States was dragging its feet and, somewhat ambiguously, endorsed a proposal from the European Union to quickly start negotiating a new international climate change treaty.

Mr. Stern’s statement to delegates from more than 190 nations at the annual climate conference was disrupted by a 21-year-old Middlebury College junior, Abigail Borah, who told the assembly that she would speak for the United States because Mr. Stern had forfeited the right to do so.