UN Secretary-General Ban Ki-moon (Oct 2009 file photo)United Nations Secretary-General Ban Ki-moon says world leaders are not likely to agree to a new climate change pact at next month's summit in Copenhagen.

Mr. Ban spoke in London Tuesday as African nations walked out of pre-summit U.N. climate talks in Barcelona, Spain. He told reporters that heads of state at the Copenhagen meeting may not be able to agree on all the details of a pact to replace the 1997 Kyoto Protocol.  

But the U.N. chief said he is still "reasonably optimistic" that the Copenhagen gathering will be an important milestone in reaffirming the commitment to a new global warming treaty.

In Barcelona, several meetings were canceled Tuesday at the U.N.-sponsored climate talks, after some African delegates pulled out in protest. The African critics say developed countries have set their carbon-cutting targets too low.

Critics from developing countries also want wealthy nations to bear more of the financial and political responsibility for reducing carbon gas emissions. They argue that they are the most vulnerable to global warming, yet the least responsible for the carbon buildup in the atmosphere that causes it.

In Washington Tuesday, U.S. President Barack Obama hosted European Union officials looking to develop momentum for a new global climate treaty to replace the Kyoto Protocol. That protocol, which expires in 2012, set a series of anti-pollution targets aimed at reducing carbon emissions by 20 percent from 1990 levels.

President Obama and congressional Democrats have pledged support for a new treaty. However, Senate Republicans on Tuesday boycotted the start of committee debate on a bill to curb carbon gas emissions. It remains far from clear when and if U.S. legislation supporting a new global treaty will be approved.

Some information for this report was provided by AFP and AP.