ROME (AP) — International food prices rose sharply in July after three months of decline, driven by a spike in grain and sugar prices, a United Nations report said Thursday.

Severe drought gripping the U.S. Midwest has sent corn prices soaring by almost 23 percent and expectations of worsened crop prospects in Russia because of dry weather sent world wheat prices up 19 percent.

The United States is the world's No. 1 exporter of corn, soybeans and wheat and the price hikes are expected to be felt across the international marketplace, hurting poor food-importing countries, said a study by British charity Oxfam issued on the eve of the U.N. report.

The Rome-based U.N. Food and Agriculture Organization said in its monthly price report on Thursday that its index climbed 6 percent in July although it was well below its peak reached in February 2011.

The agency keeps a close watch on volatile global prices because spikes in the prices of staple foods have led to riots in some countries in recent years

With corn, wheat and sugar prices sharply up, a bright note was that international rice prices remained mostly unchanged, as were dairy prices.

The agency's meat price index also fell.

Oxfam said "millions of the world's poorest will face devastation" from the rising prices. It declared that the days of cheap food are gone.

"This is not some gentle monthly wake-up call - it's the same global alarm that's been screaming at us since 2008," Oxfam spokesman Colin Roche said after latest figures were released.

"These new figures prove that the world's food system cannot cope on crumbling foundations. The combination of rising prices and expected low reserves means the world is facing a double danger," Roche said.