Rice prices have surged to a 20-year high in the latest sign of global food inflation, creating policy headaches in Asia, where more than 2.5 billion people depend on cheap and abundant supplies of the grain. Thai rice prices, a global benchmark, surged last week above the level of $500 a tonne for the first time since at least 1989, according to the United Nations Food and Agriculture Organisation, prompting importing countries to seek assurances on supplies. Robert Zeigler, director at the International Rice Research Institute in Manila, said policymakers should be concerned. "If history is any indicator, we should be worried because rice shortages have in the past led to civil unrest,' he said. US rice in Chicago, the benchmark for the world's fourth-largest exporter of the grain, jumped on Monday to a record $18.10 per hundredweight ($400 per tonne)