China is taking unprecedented action to address global warming and reduce its energy intensity, increasing chances for a new climate change agreement later this year, top U.S. officials said on Thursday.

China recently passed the United States as the biggest emitter of greenhouse gas emissions and together the two countries account for 42 percent of the world's emissions.

Chinese leaders agreed unanimously that global warming was a very serious problem, Energy Secretary Steven Chu told reporters after meeting Chinese officials, including Premier Wen Jiabao.

I am greatly encouraged by what I saw on this trip, he said.

U.S. Commerce Secretary Gary Locke also gave a strong endorsement of China's seriousness in reducing greenhouse gases.

The Chinese are taking unprecedented action , Locke said. They are a model for developing countries around the world.

Locke and Chu, both Chinese-Americans, were in Beijing for talks aimed at boosting cooperation between the two countries in fighting global warming.

Their trip came as Congress considers legislation to curb emissions in the United States.

Chinese officials raised concern during the meetings over a act recently passed by the U.S. House of Representatives allowing the government to assess carbon tariffs on imports from countries that fail to meet U.S. greenhouse gas standards, Locke said. China's Ministry of Commerce has criticised the tariffs.

During the visit, the two countries launched a $15 million joint research project aimed at creating more energy-efficient vehicles and buildings and reducing carbon dioxide emissions from burning coal.

Coal accounts for almost 80 percent of electricity generation in China, compared to about 49 percent in the United States.

Many experts believe cooperation between the two countries is essential for negotiations on a new climate treaty to succeed at a meeting this December in Copenhagen.

Chu said he was optimistic about the Copenhagen meeting, but gave no details about commitments or concessions by either side.

Climate concerns are expected to be high on the agenda when U.S. President Barack Obama visits China for a summit meeting with President Hu Jintao later this year.

Chu said the two countries could achieve big greenhouse gas reductions by working together to improve the energy efficiency of buildings, which now consume about 40 percent of the world's energy and account for nearly half of greenhouse gas emissions.

At least 30 percent of that pollution could be eliminated at no net cost by upgrading old buildings and using modern equipment in new buildings, Chu said. Building efficiency could ultimately be raised by three to five times, he said.

China is expected to build the equivalent of the entire U.S. building stock in the next 15 years, making it a tremendous laboratory for the two countries to work together on energy efficient designs, Chu said.

Frictions over the huge U.S. trade deficit with China were also on display during the trip.

In a speech on Wednesday, Locke praised steps China has taken to boost its domestic economy but called on Beijing to help revive global growth by opening its market to more foreign goods and services and moving to a more flexible exchange rate regime.

The United States contends that Beijing keeps its yuan at an artificially low value compared to the dollar, giving Chinese exports an unfair price advantage.

With the U.S trade deficit with China last year hitting a record $266 billion, many U.S. lawmakers have called for a more aggressive tack to protect American companies from widespread Chinese government subsidies and other unfair trade practices.

Climate change China