UN chief Ban Ki-moon has urged world leaders to tackle the "interconnected challenges" of climate change, rising food prices and development.
Speaking at a summit of the Group of Eight (G8) industrial countries, he said high food prices were "turning back the clock on development gains".
Spiralling oil costs are also high on the agenda at the meeting in Japan.
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The three-day summit is being held at the resort town of Toyako, on the northern Japanese island of Hokkaido.
As the meeting began, Mr Ban urged G8 leaders to help tackle the food crisis by delivering "the full range of immediate needs, including food assistance as well as seeds, fertiliser and other inputs for this year's planning cycle".
He also told reporters governments should commit to long-term agricultural investment and lift export restrictions "in particular for humanitarian purposes".
Mr Ban called on G8 leaders to work towards a new agreement on climate change, which he said was already posing major challenges in Africa.
Leaders from G8 nations - Britain, Canada, France, Germany, Italy, Japan, Russia and the United States - will be joined by counterparts from some 15 other countries, including seven African states.
The impact on the global economy of price rises and other shocks such as the credit crunch have eclipsed other concerns, correspondents say.
EU action
BBC political editor Nick Robinson, who is travelling with the UK prime minister, says Gordon Brown will join other leaders in calling for the doubling of food production in Africa.
The EU has already been spelling out plans to alleviate the food crisis.
European Commission President Jose Manuel Barroso told reporters on the sidelines of the summit that the proposed 1bn euro ($1.6bn; £800m) fund to help poor farmers in developing countries would come from unused EU subsidies.
It could help improve farmers' access to seeds and fertilisers, and could provide "safety net measures for the most vulnerable", he said.
The G8 leaders may also face tough questions on aid commitments to Africa.
Three years ago they promised to double aid to the continent by 2010 - but campaigners say they are falling far short of that target.
As well as discussing development issues in Africa, the G8 leaders have been raising Robert Mugabe's controversial re-election in Zimbabwe last month.
US President George W Bush said: "I am extremely disappointed in the elections which I labelled a sham election."
Tanzanian President Jakaya Kikwete, who is also head of the African Union, said the whole continent shared President Bush's concerns but that there was some disagreement about what to do about it.
President Kikwete called for a unity government, and said he was optimistic that "as friends at the end of the day we'll come to an understanding".
Common ground
A number of other bilateral meetings are taking place on the sidelines of the summit.
Mr Bush, attending his last G8 summit, and Russian President Dmitry Medvedev, attending his first, made little progress on the issue of the US plan for missile defence installations in the Czech Republic and Poland.
The two leaders instead cited areas where they had found common ground: preventing Iran from obtaining nuclear weapons and ending North Korea's nuclear weapons programme.
Hundreds of protesters again marched through Sapporo on Sunday, the city closest to the venue, to demand G8 leaders take action on global warming, poverty and rising food prices. The demonstration, which followed a similar protest on Saturday, was heavily policed and ended peacefully.
(BBC)
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