America, the biggest emitter of gases blamed for global warming, said it will contribute to the next round of emissions cuts, a first step to setting limits since rejecting the Kyoto Protocol six years ago.

“We will also come through with what we believe will be our contribution” to limits that will be set during talks through next year, Harlan Watson, the senior climate negotiator for the U.S. Department of State, said yesterday at a press conference in Vienna. He didn’t say by how much America would curb its emissions.

The 1997 Kyoto Protocol sets limits for reducing emissions through 2012 by an average of 5% from 1990 levels. The European Union and 36 industrialized nations were bound by those targets. The treaty doesn’t assign targets to developing nations such as China and India, and America also rejected the accord in 2001. The next round starts after 2012. “The acknowledgement that they will make cuts is new,” said Angela Leford Anderson, the climate spokeswoman for the National Environment Trust, a Washington-based lobby group. “I heard a very different tone from Mr. Watson,” compared with the previous American stance, she said in an interview in Vienna, where the United Nations is holding climate talks this week.

President Bush rejected the 1997 U.N. protocol, citing costs to the American economy of mandatory caps and China’s exclusion from them. In May, Mr. Bush called for a new round of global talks to set goals. Negotiators from the world’s biggest economies will attend those talks in Washington next month.

Preserving the climate is “is going to require tremendous cuts, well beyond 50% in a number of countries, not just in developed countries, but in developing countries,” Mr. Watson said. China may overtake America as the biggest emitter as early as next year, according to the International Energy Agency.

Ms. Leford Anderson wants America to make a binding agreement to reduce its emissions to 1990 levels by 2020. Under emissions trading, nations can either cut emissions or buy credits from projects in developing nations to counter any output that goes over target.

“To be more credible, the U.S. needs to set quantitative objectives,” Colette Lewiner, the global leader of energy, utilities, and chemicals for management consultants Cap Gemini, said yesterday from Paris. “Unless that, it’s nice words.”

The European Commission, the regulatory arm of the 27-nation European Union, wants developed nations to cut their greenhouse gases by 30% by 2020 compared with 1990 levels and to half of 1990 levels by 2050.

The commission is pressing developed nations to curb emissions in a bid to prevent temperatures from rising more than 2 degrees Celsius. Under one commission proposal, developing nations would be allowed to emit more as their economies grow, peaking around 2020, according to a January 10 statement.

Mr. Watson said Tuesday some of the world’s developing nations make a good case for being allowed to increase their emissions.