WASHINGTON - A bill to cap U.S. greenhouse-gas emissions - hailed on Capitol Hill as a historic breakthrough - went over with a soft thud this week during international negotiations, criticized as inadequate for the climate and unfair to poor countries.

The bill, which passed a House of Representatives committee last month, is regarded as the most serious effort yet to reduce U.S. contributions to climate change. But at a U.N.-led conference in Bonn, Germany, and at a summit of mega-emitters America and China in Beijing, it was derided by some environmental groups and foreign governments for a lack of ambition.

The bill's target for reducing emissions is "unacceptable to China," said Pan Jiahua, an official at a think tank affiliated with the Chinese government and a member of the Chinese government's advisory panel on climate change. "It is much too low."

That kind of reaction revealed the vastness of the work ahead, as countries seek to work out a new climate treaty by December.

This week, there was mainly posturing and gridlock - with the United States promising a first step, and others saying the situation requires a long jump.

The two sets of talks this week marked a kind of international coming-out for the U.S. bill, which passed the House Energy and Commerce in May.

As written now, it calls for a 17 percent cut in U.S. greenhouse-gas emissions, compared with 2005 levels, by 2020. It would accomplish this by a "cap-and-trade" system, which sets a national emissions limit and requires companies to amass buy-able, sell-able credits for all the gases they emit.

The bill has already been watered down, its central goal reduced from 20 percent, to please representatives of states with heavy-polluting coal and manufacturing industries. And it may get more watery: The bill must pass through several other House committees, the House floor, and the Senate.

China and other developing countries say that the United States and other rich nations have a moral duty to cut their emissions sharply since their smokestacks have been emitting greenhouse gases for far longer.

The United States and China together produce about 40 percent of the world's greenhouse-gas emissions.