Three fifths of the continental United States was in a moderate to exceptional drought, said the U.S. Department of Agriculture ( USDA) in a statement. Besides, temperatures have topped 100 degrees Fahrenheit (close to 40 Celsius) from time to time in many parts of the United States since June, especially in the central plains.

Due to damage and losses caused by the lack of rain and excessive heat, the USDA has designated 1,297 counties across 29 states as disaster areas in the 2012 crop year. It made them eligible for assistance like emergency loans with interest rates as low as 2.25 percent to 3.75 percent.

The United States is the world's major producer of grains and meat, and the damage to crops and to grazing land for livestock eventually will push U.S. and global food prices higher.

Corn price on the Chicago Board of Trade, a global corn price reference, once hit 7.89 dollars a bushel on July 17, the highest level in the past 13 months. Some market observers said it may reach a record 8.50 dollars a bushel, which in turn may cut exports of the U.S. most valuable crop.

Vilsack said prices for beef, poultry and pork probably will rise later this year and increases in the cost of processed foods will hit in 2013. Before the drought, Vilsack said the department was estimating food prices would go up between 2.5 percent and 3.5 percent this year.

But he added consumer food prices should not rise immediately, since the original farm products costs were just a small proportion of a final supermarket bill, and in any case it will take a while for the crisis to reach store shelves.