It appears to have nearly finished the work to make the launch pad 67 meters tall, a government source, speaking on condition of anonymity, told Yonhap news agency.
"We believe that the North will use the extended launch site in Dongchang-ri to fire a long-range missile longer than the Unha-3," the source said. "We think (the North) will carry out a provocation around the 70th anniversary of the foundation of the Workers' Party on October 10," he said.
Military and intelligence officials believe the new launch pad is now capable of firing long-range missiles twice the size of the 30-meter-long Unha-3, which was used to put a satellite into orbit in December 2012.
North Korea is banned under UN Security Council resolutions from carrying out any launch using ballistic missile technology, the Yonhap report said.