"Before it is too late we must take action."

The UN chief said he had been encouraged by the response to the summit he has called in New York for Sept. 24, with some 80 heads of state and government among 154 speakers listed so far. The event will precede the annual UN General Assembly gathering of world leaders.

The New York summit will not be a forum for negotiations, which will be left to a UN-sponsored meeting of environment ministers in December in Bali, Indonesia, but Ban said it could provide crucial impetus.

"What I want to achieve at the end of this particular event is a strong political message at the leaders' level for the climate change negotiations in Bali," he said.

Diplomats hope the Bali meeting will agree to start talks to find a successor to the Kyoto Protocol, the only worldwide deal on cutting carbon emissions blamed for global warming. The pact expires in 2012.

"We need to move fast and reach a bold agreement by 2009, so that it can enter into force by the end of 2012," Ban said. "We must not leave any vacuum after the expiration of the Kyoto protocol."

Ban said he saw no clash between the New York summit and a gathering of the world's biggest greenhouse-gas emitting countries that US President George W. Bush has convened in Washington on Sept. 27-28, just days later.

"I think the main objectives and principles and goals we are working are all the same," he said. "While we welcome individual measures and initiatives by any countries, all these measures and initiatives should should fit into UN efforts.

"I am encouraged that President Bush is also committed to this UN negotiating process," he said, adding that Bush had given him this assurance at a Group of Eight summit in June.

Ban also played down disputes between developed and developing countries over who should bear the brunt of carbon emission restrictions.