"At this moment it doesn't look very encouraging," said Yvo de Boer, Executive Secretary of the UN Framework Convention on Climate Change (UNFCCC).

He told AFP that the G8 nations -- France, Germany, Britain, Canada, Japan, Italy, Russia and the United States -- would probably not be able to agree on a target for the reduction of greenhouse gases by mid-century.

At the previous summit last July, host country Germany overcame fierce US resistance and coaxed the rich nations' club into agreeing to "consider seriously" the goal of at least halving worldwide emissions of greenhouse gases by 2050.

The Nobel-winning UN panel of climate change scientists, de Boer recalled, has insisted on the "absolute necessity" of a 50 percent reduction by mid-century if climate change is to remain manageable.

De Boer said it was also essential to set medium-term goals for reducing carbon dioxide, the main driver of rising global temperatures, by 2020.

"We really need that focus from the rich nations on 2020 -- it is necessary to give a strong signal to the private sector," he said in a telephone interview.

To remain relevant to the climate change debate, the G8 nations must bring developing economic giants into the discussion, de Boer suggested.

He praised Germany for launching an informal "G8 + 5" initiative last year, including China, India, Brazil, Mexico and South Africa.

"In an era when we are expecting engagement from major developing countries on an important topic like climate change, I also think you need to treat them with the respect they deserve," he said.

The G8 summit will take place on July 7-9 in northern Japan.