Street fighting broke out early on Sunday in Kharkiv as Russian troops pushed into Ukraine’s second-largest city, according to a regional official, following a wave of attacks elsewhere targeting airfields and fuel facilities that appeared to mark a new phase of an invasion that has been slowed by fierce resistance. Russian troops approached Kharkiv, about 20 kilometres (12.4 miles) south of the border with Russia, shortly after Moscow launched its invasion of Ukraine on Thursday. But until Sunday, they remained on the outskirts of the city of 1.4 million without trying to enter while other fo…