In the first five verses of chapter 3, the prophet had addressed the city in the third person (“She listens to no voice…. Her officials within her…. The Lord within her”). The prophecy was describing things about the city of Jerusalem and its inhabitants, making known that he will not withstand an unholy people. Zephaniah 3:6, though, marks a shift, where the Lord now speaks in the first person (“I have cut off nations…”) until the end of verse Zephaniah 3:13. He is going to act in light of the sins of Jerusalem.
6 “I have cut off nations; their battlements are in ruins; I have laid waste their streets so that no one walks in them; their cities have been made desolate, without a man, without an inhabitant.