Sept. 25 (Bloomberg) -- United Nations Secretary-General Ban Ki-moon heightened expectations for global climate-change talks and put increased pressure on the U.S. to help achieve a ``real breakthrough'' on cutting greenhouse-gas emissions.

Ban convened the largest gathering ever of world leaders to discuss global warming in New York yesterday and to set the agenda for the opening later this year of negotiations on a new, stronger international treaty to stem rising world temperatures.

```I have high expectations of all countries, including the United States,'' Ban, 63, said after the meeting. While he expressed optimism that the ``political will'' to forge a new accord is gaining momentum, he also said that it might be a ``long and difficult negotiation process.''

World leaders at the climate conference, joined by California Governor Arnold Schwarzenegger and former Vice President Al Gore, indirectly put the focus on the U.S. by stressing the need for the world's major emitters to agree on greenhouse-gas cuts. The U.S., the world's largest economy, is the biggest source of such emissions.

``I still see hurdles'' to reaching reduction targets, German Chancellor Angela Merkel told reporters. ``Negotiating how to break it down, defining the instruments and partitioning the shares will be very, very difficult.''

President George W. Bush vowed in June to have the U.S. participate in upcoming climate negotiations, and his administration is holding its own meeting on the topic later this week. Still, he remains opposed to the mandatory emissions cuts that Merkel and other heads of state say are necessary to combat the worst effects of a warming planet.

Talks in Indonesia

That sets the stage for a possible stalemate in December when UN countries meet in Indonesia to begin climate talks.

World leaders, UN officials and environmentalists alike say it's crucial that the U.S. as the biggest emitter sign on to any new treaty. U.S. involvement is also viewed as vital because those involved in the issue say it may spur developing nations such as China that are rapidly growing sources of pollution to agree to mandatory emission cuts.

``There's no answer to climate change without the participation of the U.S.,'' Achim Steiner, executive director of the UN Environment Programme, said in an interview.

Last night, Bush attended a dinner with other leaders hosted by Ban to discuss global warming and the steps ahead.

Seeking a `Breakthrough'

``If we don't act now, the impact of climate change will be devastating,'' Ban said earlier in the day. A ``real breakthrough'' will be needed to reach a new treaty before the current agreement, the Kyoto Protocol, expires in 2012, he said.

Schwarzenegger, 60, told the climate summit that California, which last year passed the first state law to reduce heat-trapping gases, ``is moving the U.S. beyond debate and doubt to action.'' He said, ``The time has come to stop looking back at the Kyoto protocol.''

Gore, 59, who has focused his work since leaving office on raising awareness of the global warming threat, repeated a call to have a new climate treaty in place before Kyoto expires.

``We can't continue business as usual,'' the Tennessee Democrat, who lost the 2000 presidential election to Bush, said at a UN luncheon. ``We must put a price on carbon.''

Freedom Agenda

Still, the issue likely will make up a small part of Bush's address to the UN General Assembly this morning. Instead, he will concentrate on global humanitarian concerns and advancing freedom. Specifically, he will announce new sanctions on Myanmar's military regime and urge the UN to take tougher action against the country formerly known as Burma.

Bush, 61, will call for the UN and governments around the world ``to do all they can to support a process of political change in Burma,'' National Security Adviser Stephen Hadley said.

Bush's speech today also will touch ``a little bit'' on Iran, according to Hadley.

Iranian President Mahmoud Ahmadinejad, speaking yesterday during a controversial visit to Columbia University in New York, defended his views questioning the Holocaust.

University President Lee Bollinger, under fire from protesters who said Ahmadinejad should never have been invited, referred to his guest as a ``petty and cruel dictator.''

Thousands of protesters demonstrated outside the auditorium where Ahmadinejad was speaking. Bollinger defended the invitation, saying free-speech principles in the U.S. require open debate. Ahmadinejad will address the UN today after Bush speaks.

Middle East

Bush, 61, heads into the UN meeting after affirming the U.S. commitment to an independent Palestinian state yesterday. At the same time, an administration official scaled back expectations for a Middle East peace meeting in November.

Bush had a 90-minute meeting with Palestinian Authority President Mahmoud Abbas where they discussed issues related to the planned international gathering on a peace settlement and Palestinian security changes.

The president also met yesterday with representatives of the UN, the European Union and Russia, which along with the U.S. make up the so-called Quartet on Middle East peace.

To contact the reporter on this story: Kim Chipman in New York at kchipman@bloomberg.net; Hans Nichols in New York at hnichols2@bloomberg.net.

Last Updated: September 25, 2007 00:03 EDT