The G8's member states include the US, Japan, France, Britain, Germany, Canada, Italy and Russia.
The group agreed at its 2005 summit in Gleneagles, Scotland, to double aid to Africa to $25bn a year by 2010 as part of a wider drive to alleviate global poverty.
But activists have accused some G8 countries of backtracking on those commitments.
|What is the G8?
The G8 nations are the US, UK, Canada, France, Italy, Germany, Russia and Japan.
G8 concept first emerged in mid-1970s as informal forum for the world's leading industrialised democracies when it had just six members (the G6).
Canada joined the group in 1976 and the group became the G8 when Russia formally joined in 1997.
Group has rotating presidency responsible for planning and hosting annual meetings. This year Japan chairs the meetings.
In 2005 five outreach countries were added to the forum (the G8+5), representing five leading emerging economies: Brazil, China, India, Mexico and South Africa.
UN and African Union figures indicate that less than a quarter of the pledged amount has been forthcoming.
Last month a report by the Africa Progress Panel, a group set up to monitor implementation of the Gleneagles plan, said that under current spending the G8 will fall $40bn short of its target.
The report was rebutted by Japan's foreign ministry on Sunday, which denied the G8 was failing to deliver on its promises.
Kazuo Kodama, a Japanese foreign ministry spokesman, said the report was "completely false and unfounded", although he acknowledged that Africa was well behind target on health.
Other topics up for discussion in the Japan summit include measures to tackle climate change, record oil prices and a deteriorating global economy.
But speaking ahead of the summit on Sunday, George Bush, the US president, said he viewed aid for Africa as his top priority and would hold fellow G8 leaders to account on their pledges.
"We'll be very constructive in the dialogue when it comes to the environment - I care about the environment - but today there's too much suffering on the continent of Africa, and now's the time for the comfortable nations to step up and do something about it," Bush said.
Activists have welcomed a possible accountability mechanism for future G8 summits, but criticised the grouping's lack of major financial commitments.
Oxfam, a British charity and advocacy group, said G8 members were instead trying to water down a pledge made at last year's summit in Germany to meet the Gleneagles goals.
Max Lawson, a policy adviser to Oxfam, said the Hokkaido summit was arguably the most important G8 gathering in a decade.
"The world is clearly facing multiple crises - serious, serious economic problems, both rich and poor countries. But it is poor people who suffer the most, suffering hugely from food price increases," he said.
A preliminary World Bank study released last week estimated that up to 105 million more people could drop below the poverty line due to rising food prices, including 30 million in Africa.
|Bush, right, and Medvedev met on the sidelines of the G8 meeting [AFP]
The G8 meeting has also provided the first opportunity for Bush to meet Russia's new president, Dimitry Medvedev, for the first time.
On Monday the two men held their first face-to-face talks, with Mededev reiterating Russian opposition to US plans to base part of a missile shield in eastern Europe.
"The Russian president openly expressed his serious concern about the media reports about talks between the US and Lithuania on the possible installation of anti-missile bases," Sergei Prikhodko, Medvedev's diplomatic adviser, said.
"It was said that for the Russian side this was absolutely unacceptable," Prikhodko told journalists after Bush and Medvedev met on the sidelines of the summit.