SYDNEY - The world's two biggest polluters, the US and China, will have an unprecedented chance to thrash out action on climate change at an upcoming summit in Australia, Prime Minister John Howard said Sunday.

US President George W. Bush and Chinese President Hu Jintao will be among 21 leaders at the Asia-Pacific Economic Cooperation (APEC) forum in Sydney in September, where global warming is expected to be high on the agenda.

"This will be the first and best opportunity for the two largest polluters in the world -- the United States and China -- to come together," Howard told reporters.

"At a lot of these other meetings, the Americans and the Chinese aren't sitting down together, except as part of the enormous concourse of everybody in the United Nations' ambit."

Howard said he expected there would be "a very significant discussion about climate change" at the meeting.

"I'm not suggesting we are going to solve the problem of climate change at APEC, but I do think it will be a principal point of discussion," he said.

"Having both China and the United States around the same table is a huge advantage."

Howard's conservative government has adopted a cautious approach to climate change, joining the United States in refusing to sign the Kyoto Protocol on reducing greenhouse gas emissions.

But as scientific evidence linking global warming to human activity mounts and public pressure for action grows, the prime minister has declared himself a "climate change realist" and embraced action including emissions trading.

Fellow convert Bush said last month the US was ready to take a leading role in a global bid to fight climate change but said China and India must get on board.

"The US will be actively involved, if not taking the lead, in a post-Kyoto framework, a post-Kyoto deal," Bush said on the sidelines of a Group of Eight summit in Germany.

"By 2008 the world's emitters of greenhouse gases should come together. Nothing is going to happen in terms of substantial reduction unless China and India participate."

At the G8 meeting, however, the leaders of the world's wealthiest nations were content to simply declare their intention to pursue "substantial" cuts to dangerous greenhouse gas pollution, with no actual concrete goals laid down.