[A dinghy overcrowded by migrants and refugees approaches the island of Lesbos as Greek and EU flags flutter atop a hill]By Alastair Macdonald and Gabriela Baczynska BRUSSELS (Reuters) - The EU executive concluded on Wednesday that Greece could face new border controls with other states of the free-travel Schengen zone in May if it does not fix "serious deficiencies" in its management of the area's external frontier. "If the necessary action is not being taken and deficiencies persist, there is a possibility to ... allow member states to temporarily close their borders," European Commission Vice President Vice President told a news briefing after the executive accepted a report that Athens had "seriously neglected" its obligations to fellow Schengen states. The move is part of a process in which EU governments aim to give them the option of reinstalling controls on their national borders for up to two years, once short-term derogations currently in place expire in May. Following the arrival of more than a million irregular migrants in the European Union last year, most of them via Turkey into Greece from where they trekked northward to Germany, other EU member states have instituted emergency controls on their internal Schengen borders and have warned that they might effectively suspend Athens from the passport-free travel zone.