Wheat prices surged Thursday after Russia announced a ban on grain exports, extending a rally that is driving up the cost of corn and other substitutes and underscoring the risk of a broad shock to the global food supply.

Russian Prime Minister Vladimir Putin, responding to the country's crippling drought and deadly wildfires, said on Thursday that exports will be banned from Aug. 15 until the end of the year.

Russia has become an increasingly important force in the global supply of grains and the move reignited fears that nervous governments will begin hoarding their own supplies, potentially causing a shortage. Nations that already are struggling to feed themselves are scrambling to lock in deliveries, while food companies are facing costs, as are farmers who need grain to feed livestock.

Memories of the grains shortages and price spikes of 2008, which sparked world-wide food riots, are still fresh. The world's stockpiles are still much higher than two years ago, and prices far lower, but many worry the situation will worsen.

"The situation is very, very precarious. It will have a spillover to everywhere," said Abdolreza Abbassian, secretary of the intergovernmental group on grains at the U.N. Food and Agriculture Organization in Rome.