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

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

He denied the U.S. was dragging its feet and endorsed a proposal from the European Union to quickly start negotiating a new international climate treaty.

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

"I am speaking on behalf of the United States of America because my negotiators cannot," said Borah, who is attending the conference as a representative of the International Youth Climate Movement. "The obstructionist Congress has shackled justice and delayed ambition for far too long. I am scared for my future. 2020 is too late to wait. We need an urgent path to a fair, ambitious and legally binding treaty."

Borah, of Princeton, N.J., added: "We need leaders who will commit to real change, not empty rhetoric. Keep your promises."

Scores of delegates and observers gave her a sustained ovation. Then South African authorities threw her out of the conference.

Stern smiled as if the applause were for him and then continued with his prepared remarks.

Afterward, he dismissed charges that the United States was blocking action on climate change until after 2020.

He detailed a number of domestic and international actions that the United States has taken and said he and other administration officials were working on others, such as finding ways to raise tens of billions of dollars to help poor nations adapt to a warming planet.

"Taking all of those things together, it's nonsense to suggest that what we are doing is proposing a kind of hiatus in dealing with climate change until after 2020," Stern said.

He then seemed to endorse a European Union proposal to adopt a "road map" for future discussions leading to a formal climate-change treaty to be completed by 2015 and to take effect in 2020.

He had previously given lukewarm support to the plan, saying only that the United States was open to a "process" for a future agreement.

He said the EU had called for a road map "that the U.S. supports."

"We are strongly committed to promptly starting a process to move forward on that," he added, although he immediately qualified that statement by saying any resulting agreement may or may not be legally binding.

The Europeans and a large majority of smaller nations are adamant that any future accord be legally binding, while China, India, the United States and several other major emitters of greenhouse gases have attached some difficult conditions to participation in any mandatory agreement.