By Joseph Nasr and Markus Wacket BERLIN (Reuters) -Pressure on Europe to secure alternative gas supplies increased on Thursday as Moscow imposed sanctions on European subsidiaries of state-owned Gazprom a day after Ukraine stopped a major gas transit route. Gas prices surged, with the key European benchmark gaining 12% as buyers were unsettled by the mounting threats to Europe’s supply given its high dependence on Russia. Moscow has already cut off supply to Bulgaria and Poland and countries are racing to fill dwindling gas reserves before winter. Russia imposed sanctions late Wednesday mainly…