By Marja Novak LJUBLJANA (Reuters) - The migrant crisis engulfing Europe threatens to reignite conflicts between former Yugoslav republics which fought each other during the 1990s, Slovenia's prime minister said on Tuesday. Hundreds of thousands of migrants, many of them fleeing Syria's civil war, have taken the Balkan route from Greece to western Europe this year, putting great strain on the finances and infrastructure of transit countries, including the ex-Yugoslav states of Macedonia, Serbia, Croatia and Slovenia. The situation worsened when Hungary, an eastern outpost of the European Union's passport-free Schengen zone, shut its southern border to the migrants on Oct. 16, diverting the flow of around 135,000 people westwards from Croatia into Slovenia.