Five of the biggest emerging economies have urged leading industrial nations to do more to combat climate change.
Mexico, Brazil, China, India and South Africa challenged the Group of Eight countries to cut their greenhouse gas emissions by more than 80% by 2050.
The so-called G5 countries threw down the gauntlet in a statement before they joined the G8 summit in Japan.
Earlier, the G8 restated a lower target of 50% cuts over the same period, which environmentalists said was "pathetic".
The joint statement from the G5 developing nations said: "It is essential that developed countries take the lead in achieving ambitious and absolute greenhouse gas emission reductions."
The five nations also urged developed countries to commit to an interim target of a 25-40% cut below 1990 levels by 2020.
The chairman of the UN's Intergovernmental Panel on Climate Change, RK Pachauri, said developed countries should show leadership.
"They should get off the backs of India and China," he told reporters in the Indian capital, Delhi.
"They should say: 'We'll assist you to move to a pattern of development which is sustainable, low in terms of emissions intensity. But we as the richest nations are willing to take the lead and we affirm our commitment to do so.'"
The US has refused to set any interim targets for cutting emissions.
South Africa said the G8 statement was a "regression", criticising the lack of firm targets to achieve sufficient cuts in emissions.
The G8 also voiced concern about soaring oil and food prices, and Zimbabwe, and pledged to speed up aid to Africa. But climate change has been one of the stickiest issues tackled at the summit in Toyako, northern Japan.
The five-page communique by the G8 - Britain, Canada, France, Germany, Italy, Japan, Russia and the US - repeats last year's "vision" to reach the target of cutting emissions by at least 50% by 2050.
But it added: "This global challenge can only be met by a global response, in particular, by the contributions from all major economies."
The statement also acknowledges that to make progress, G8 countries - which account for nearly 60% of the world economy - have to take the lead through interim goals and national plans.
It strengthens last year's pledge to "seriously consider" emission cuts, but the G8 said this was only achievable if emerging economies also committed themselves.
Kim Carstensen, of the WWF's Global Climate Initiative, said: "If after a year's work all you have is a 'shared vision' instead of 'seriously considering', it's pretty pathetic."
But German Chancellor Angela Merkel told reporters it was a "significant step forward" and British Prime Minister Gordon Brown said it was "major progress".
US President George W Bush's assistant for international economic affairs, Dan Price, said it represented "substantial progress".
The BBC News website's environment correspondent, Richard Black, says the G8's joint statement, in fact, is exactly what leaders of nearly 200 countries signed up to in the original UN climate change convention agreed at the 1992 Earth Summit.
He says that if re-stating a 16-year-old commitment is progress, then this is clearly a success.
The EU had wanted the G8 to confirm that the 50% cut would be measured from 1990 levels of CO2 - as agreed under the Kyoto climate protocol.
But when the question was raised in a press conference Japanese Prime Minister Yasuo Fukuda said the cuts would be measured from "current levels".
Our correspondent says this is significant in several ways, not least because a 50% cut from now is worth far less than a 50% cut from 1990 levels.
The G8 has issued statements on several key issues:
- Aid and development: Commitment to fulfil earlier pledge to raise annual aid levels by $50bn by 2010, of which $25bn is intended for Africa
- Global food prices: Call for countries with sufficient food stocks to release reserves to others struggling to cope with rising costs
- Biofuels: Pledge to ensure biofuel policies are compatible with food security.