The Group of Eight major powers agreed Tuesday to at least halve global carbon emissions by 2050 in what leaders hailed as a step forward, but developing nations rejected as an "empty slogan".

After two days huddled in the Japanese mountain resort of Toyako, leaders of the world's eight most powerful economies also voiced concern about soaring oil and food prices, pledged to speed up aid to Africa and threatened to take further action against Zimbabwe's regime.

But the most contentious issue before them was climate change, with US President George W. Bush standing firm on his stance that developing countries must take action before rich nations would budge.

The leaders of the G8 -- Britain, Canada, France, Germany, Italy, Japan, Russia and the United States -- said they shared a "vision" of reducing emissions by at least 50 percent by 2050.

The summit last year in Heiligendamm, Germany had agreed only to "seriously consider" cuts in the greenhouse gas emissions blamed for heating up the planet.

"This is a significant step forward from Heiligendamm," German Chancellor Angela Merkel told reporters. "This means that the international community will no longer get off the hook."

But UN climate chief Yvo de Boer complained that elements on how rich nations would cut their own emissions in the next dozen years -- a period deemed critical by scientists -- was "completely missing."

The G8 nations said they would each set their own interim targets for curbing greenhouse gas emissions for an unspecified amount of time after the Kyoto Protocol's obligations expire in 2012.

"If it's not clear that rich nations are going to lead, then why should poor nations follow?" de Boer, who heads negotiations that aim to reach a post-Kyoto treaty by the end of next year, told AFP by telephone from Bonn.

In a nod to Bush, the G8 leaders called on major developing nations to join them in cutting emissions.

"In our view, and in the view of the leaders in the room, this represents substantial progress from last year," said Dan Price, Bush's assistant for international economic affairs.

Japanese Prime Minister Yasuo Fukuda had pleaded for this year's summit not to backtrack on earlier pledges on global warming, which UN scientists warn could put entire species at risk unless it is curbed by later this century.

"It's been a long road getting here. We had some very tough negotiations," Fukuda told reporters.

But the G8 leaders can expect another difficult round of talks Wednesday when they are joined by leaders of the developing world.

Chinese President Hu Jintao, Indian Prime Minister Manmohan Singh and three other leaders of developing nations met in the nearby city of Sapporo Tuesday and urged that rich countries cut emissions by 25 to 40 percent by 2020 from 1990 levels.

South Africa's environment minister Marthinus van Schalkwyk said the G8 had made no progress at all in stabilising the planet.

"As it is expressed in the G8 statement, the long-term goal is an empty slogan without substance," he said.

The G8 deal was full of ambiguity. Senior Japanese official Koji Tsuruoka said the long-term goal should be seen as a "political vision" without a clear base year and that it is not legally binding.

French President Nicolas Sarkozy appeared to disagree, saying: "We have reached a binding level. That's real progress."

But one of the world's most respected climate scientists, James Hansen, slammed the summit's outcome as "worse than worthless."

The leaders "are taking actions that guarantee that we deliver to our children climate catastrophes that are out of our control," he told AFP.

Daniel Mittler, a climate change expert at Greenpeace International, agreed, saying that "instead of action, the world got flowery words."

"The Texas oilman has once again prevented the G8 from undergoing the energy revolution it needs," Mittler said. "Bush is a lame duck, so who cares what he thinks about 2050?"

The United States is the only major industrial nation to shun the Kyoto Protocol. Bush argues that it is unfair because it makes no demands of growing emerging economies such as China and India.

But both major candidates to succeed Bush, John McCain and Barack Obama, have pledged stronger action on global warming, including forcing domestic industry to cut emissions in the world's largest economy.

The G8 leaders also issued a statement warning that soaring oil and food prices pose a "serious challenge" to world economic growth and called for boosted crude oil production capacity.

In another area of contention, the leaders set a timeframe of five years to commit 60 billion dollars to Africa to help fight AIDS, malaria and tuberculosis.

But rocker turned aid activist Bob Geldof, who personally lobbied leaders here, said Africa needed money immediately, not pledges.

"I think the G8 have a different view of commitment to what I have," Geldof told AFP. "I'm disappointed although I must say we didn't expect much from this G8."