By Andrea Shalal and David Lawder WASHINGTON (Reuters) -Russia’s war in Ukraine is to blame for exacerbating “already dire” world food insecurity, with price and supply shocks adding to global inflationary pressures, U.S. Treasury Secretary Janet Yellen said on Tuesday. Even before the war, over 800 million people – or 10% of the global population – were suffering from chronic food insecurity, Yellen said, and estimates showed higher food prices alone could push at least 10 million more people into poverty. Yellen told a high-level panel countries should avoid export bans that could further bo…