POZNAN, Poland, Dec 5 (Reuters) - The U.N.'s top climate change official defended a U.N. project that promotes clean energy in developing nations from U.S. criticisms on Friday, but conceded the scheme was in need of an upgrade.

The investigative arm of the U.S. Congress, the General Accountability Office (GAO), criticised on Tuesday the U.N. scheme that has channelled investments to nations such as China and India. It said its impact in limiting greenhouse gases was "uncertain."

"There are a number of critical points in the report...but I see it as offering a number of constructive suggestions," Yvo de Boer, head of the U.N. Climate Change Secretariat, said on the sidelines of Dec. 1-12 talks in Poznan, Poland.

The GAO criticised the Clean Development Mechanism (CDM), under which rich nations can invest, for instance, in windmills in Ecuador or in cutting emissions from a factory in China and claim credits to balance against domestic emissions.

"It's not as though we're printing money in the garage," de Boer said, adding there were checks and balances in place to ensure that emissions cuts were real.

"There is room for improvement and there are projects that perhaps make it through that shouldn't have...we are in a learning experience," he told reporters.

De Boer said governments were discussing ways in Poznan to improve the CDM as part of a long-term treaty to combat climate change due to be agreed in Copenhagen next year.

The GAO report said carbon offsets, allowing polluters to compensate for their emissions elsewhere, "involve fundamental trade-offs and may not be a reliable long-term approach to climate change mitigation."

The report comes as the United States, the world's top greenhouse gas emitter after China, is expected to form its own carbon market.

U.S. President-elect Barack Obama hopes to form a market by putting limits on greenhouse gas emissions and tightening those limits over time. The aim is to bring U.S. emissions back down to 1990 levels by 2020, and reduce them by 80 percent by 2050.

-- For Reuters latest environment blogs click on: http://blogs.reuters.com/environment/ (Editing by Marguerita Choy)

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