BUDAPEST - Hungary plans to build a second fence on its southern border with Serbia that would enable it to keep out any major new wave of migrants, Hungarian Prime Minister Viktor Orban said on Friday.Orban said the new barrier, to be built alongside the existing one, would strengthen defenses to respond if Turkey's policy on migration changed. If that happened, hundreds of thousands could appear at Hungary's border, he told public radio."Technical planning is under way to erect a more massive defense system next to the existing line of defense which was built quickly (last year)," Orban said.Orban said Hungary had to prepare for the eventuality of a deal between Turkey and the European Union to clamp down on migration into Europe via the Balkans unravelling."Then if it does not work with nice words, we will have to stop them with force, and we will do so," Orban said.A razor-wire fence built along Hungary's southern border with Serbia and Croatia has sharply reduced flows.Last year hundreds of thousands of migrants moved up from the Balkans towards northern Europe. That flow has since been reduced to no more than a steady trickle.