ISTANBUL — In a sign of escalating frustration in Turkey after days of cross-border shelling with Syria, Prime Minister Recep Tayyip Erdogan lashed out at the United Nations’ inaction in Syria with some of his strongest comments yet, saying world powers are repeating the mistakes they made in Bosnia in the 1990s.

‘‘This negligence 20 years ago was explained by the international community being caught unprepared in dealing with the issues of the post-Cold War era,’’ Erdogan said at an international conference in Istanbul. ‘‘Well, how can the injustice and weakness displayed in the Syrian issue be explained today?’’