GENEVA (Reuters) -A World Food Programme (WFP) official said on Friday that food supply chains in Ukraine were collapsing, with a portion of infrastructure destroyed and many grocery stores and warehouses empty.

       "The country's food supply chain is falling apart. Movements of goods have slowed down due to insecurity and the reluctance of drivers," Jakob Kern, WFP Emergency Coordinator for the Ukraine crisis, told a Geneva press briefing by videolink from Poland.

       He also expressed concern about the situation in "encircled cities" such as Mariupol, saying that food and water supplies were running out and that its convoys had been unable to enter the city.

       WFP buys nearly half of its wheat supplies from Ukraine and Kern said that the crisis there since the Russian invasion on Feb. 24 had pushed up food prices sharply.

       "With global food prices at an all-time high, WFP is also concerned about the impact of the Ukraine crisis on food security globally, especially hunger hot spots," he said, warning of "collateral hunger" in other places.

       The agency is paying $71 million a month extra for food this year due to both inflation and the Ukraine crisis, he said, adding that such an amount would cover the food supplies for 4 million people. "We are changing suppliers now but that has an impact on prices," he said. "The further away you buy it, the more expensive it gets."

       (Reporting by Emma Farge; Editing by Hugh Lawson and Barbara Lewis)