A statement from North Korean scientists and technicians said they found "a technical deficiency in the first-stage control engine module of the rocket carrying the satellite" and decided on the extension, South Korea's Yonhap News Agency reported Monday.
The statement, first reported by North Korea's Korean Central News Agency, emphasized the country was "pushing forward" with the rocket launch and the "final stage" of preparations had begun.
The launch was originally scheduled in a Dec. 10-22 window.
Satellite images last week showed that a rocket's three stages were at the launch pad in the nation's northwestern tip.
Observers told Yonhap North Korea likely will fire the rocket as soon as it fixes the problems, despite international pressure to drop the plan. North Korea unsuccessfully fired a rocket in April.
North Korean officials said the launch is for peaceful purposes but many nations consider it a ballistic missile test that is banned under U.N. resolutions.
South Korean military officials said they will closely monitor the planned rocket launch.