World food prices posted their biggest monthly rise for four years in June, buoyed by a surge in sugar and increases for most other edible commodities, the United Nations food agency said Thursday.Food prices have been gaining ground since hitting a near seven-year low in January after four straight annual declines, and the United Nations Food and Agriculture Agency (FAO) now expects them to be stable for the next decade.The 4.2 percent gain from May was the fifth increase in a row for the index, which measures monthly changes for a basket of cereals, oilseeds, dairy products, meat and sugar. Food prices on international markets are now just 1 percent below the same month last year, the FAO said.