By Paul Taylor and Alastair Macdonald ATHENS/BRUSSELS (Reuters) - Greece hit back on Wednesday at threats from some EU states to suspend it from the Schengen zone of open border travel because of its failure to control large numbers of migrants entering Europe. Some central European officials, most prominently Slovak Prime Minister Robert Fico, have suggested excluding Greece from Schengen. Diplomats and European Union officials say some governments have raised the possibility informally but it would be a largely symbolic move, with little impact on migration.