SEOUL — North Korea defied the likelihood of more sanctions by the United Nations Security Council by declaring the successful launch of a long-range rocket on Wednesday, demonstrating that the government of its new leader, Kim Jong Un, is pressing ahead to master the technology needed to deliver a nuclear warhead on intercontinental ballistic missiles.

The Unha-3, or Galaxy-3, rocket blasted off from the Sohae Satellite Launching Station in Tongchang-ri on North Korea’s western coast near China on Wednesday morning, a spokesman for South Korea’s Defense Ministry said.