Rising food prices could cause political instability worldwide, the UN's top humanitarian official said Tuesday, as clashes over food costs in Haiti and Egypt continued for a second day.

Pointing to a 40 per cent average rise in food costs worldwide since mid-2007, John Holmes, the United Nations under-secretary-general for humanitarian affairs and emergency relief co-ordinator, said the trend is likely to exacerbate both the incidence and depth of food insecurity worldwide.

"The security implications [of the food crisis] should also not be underestimated as food riots are already being reported across the globe," he said.

Egypt has been rocked by two days of unrest as thousands protest the country's worsening economy and rising prices. The cost of cooking oil, rice and other staples have doubled since January, while the country of 76 million is suffering a shortage of government-subsidized bread.

UN peacekeepers in Haiti clashed with a crowd that gathered outside the presidential palace Tuesday, the second day of protests in the capital, Port-au-Prince, over food scarcity and costs. 

The impact of climate change has worsened the food problem, Holmes said, as the number of recorded natural disasters has doubled from an average of 200 a year to 400 because of "extreme weather" over the last 20 years.

The rising price of fuel, particularly diesel fuel used to transport food, is also adding to the issue by prompting a simultaneous increase in the cost of food, Holmes said. 

"Compounding the challenges of climate change in what some have labelled the perfect storm are the recent dramatic trends in soaring food and fuel prices," he said.

The UN's World Food Program has said it is short $500 million US needed to feed 89 million hungry people this year. Meanwhile, consumers should expect high food costs for at least another 10 years, according to the UN's Food and Agriculture Organization.

"It's not likely that prices will go back to as low as we're used to," Abdolreza Abbassian, economist and secretary of the Intergovernmental Group for Grains for the FAO, said last month.

"Currently if you're in Haiti, unless the government is subsidizing consumers, consumers have no choice but to cut consumption. It's a very brutal scenario, but that's what it is."

With files from the Associated Press