BELGRADE, Serbia — With freezing weather setting in, tens of thousands of migrants surging across Europe could face even more hurdles after European Union leaders pledged to stem their flow by introducing tighter border controls. EU leaders committed at a weekend summit to helping the Balkans handle the flow of people making their way through the region en route to more prosperous countries. The EU and Balkan leaders meeting in Brussels committed to strengthen the borders of Greece, and they pledged to boost the capacities of reception centers in Greece and along the Balkans route to shelter 100,000 more people. “This is a step in the right direction and now it is crucial to respect the commitments,” said Slovenian Prime Minister Miro Cerar, whose tiny Alpine nation has been overwhelmed since Hungary put up a fence on the border with Serbia and Croatia, diverting the flow to Slovenia.