The aim, agreed at a G8 summit in the central Italian town of L'Aquila, is to cut overall world emissions by 50 percent in order to limit global warming to no more than two degrees Celsius, a declaration said.

Despite G8 backing for a 50 percent cut in emissions globally, a broader group of major polluters, including many developing countries such as China and India, dropped a pledge earlier to halve their pollution by 2050.

"We recognise the broad scientific view that the increase in global average temperature above pre-industrial levels ought not to exceed two degrees Celsius," the G8 leaders said.

"Because this global challenge can only be met by a global response, we reiterate our willingness to share with all countries the goal of achieving at least a 50 percent reduction of global emission by 2050," they added.

The 50 percent target was first put in writing at the G8 summit in Japan last year, but according to Japanese government spokesman Kazuo Kodama, the big eight leaders have struggled to convince major emerging economies to join in.

"You remember G8 achieved a consensus within G8 that G8 will embrace a long-term gas reduction target by 2050?" he said on the margins of the summit, speaking on behalf of Prime Minister Taro Aso.

"Our leaders tried hard to convince the emerging economies in the out-reach five. They didn't agree. This year we hoped the MEF countries as a group would also embrace this target but I haven't heard yet there's any agreement."

The Major Economies Forum (MEF) represents the G8 members plus the most important emerging economies, which all together generate 80 percent of global greenhouse gas emissions.

US President Barack Obama and other leaders face mounting pressure to make ambitious commitments to cut greenhouse gas emissions with the clock ticking ahead of a key December meeting in Copenhagen to set international targets.

Swedish Prime Minister Frederik Reinfeldt, whose country holds the European Union's rotating presidency, stressed agreeing an over-arching target to limit global warming to two degrees Celsius by 2050 would be crucial.

"The G8 and the Major Economies Forum must endorse the goal of limiting global warming to no more than two degrees," Reinfeldt told journalists shortly before the summit got under way.

Temperatures have already risen by about one degree Celsius worldwide over the last century.

European Commission chief Jose Manuel Barroso said countries were much closer now to securing agreement on the target than they were a year ago, because the United States was prepared to back it.

"For the first time, now under President Obama, ... the United States accepts the reference of two degrees," he said.

The US Congress narrowly approved the "Clean Energy Act" last month, helping to restore US leadership in the battle against climate change after a go-slow posture adopted by Obama's predecessor George W. Bush.

The bill, now before the Senate, aims to reduce greenhouse gas emissions by 17 percent from 2005 levels by 2020, and 83 percent by 2050.

The 27-nation EU is aiming to cut greenhouse gas emissions by 20 percent from 1990 levels by 2020, increasing to 30 percent if other big polluters make ambitious commitments in Copenhagen.

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