In an escalation of tensions over immigration, the Austrian interior minister threatened on Wednesday to sue Hungary if the country refused to take back migrants crossing the shared border between the two countries. Austria's government has repeatedly accused Hungary of allowing migrants to enter its country in violation of European Union asylum rules that say migrants must stay in the first EU country they enter. Hungary, which will hold a referendum next month on whether to accept an EU-wide asylum quota, has responded by saying that most refugees enter its territory from other EU states.