The 67-metre (220-ft) tower will be able to accommodate the launch of a longer-range missile than the rocket test-fired in December 2012. That launch was widely considered successful in putting an object into space orbit, the Yonhap report said.

North Korea is banned under U.N. Security Council resolutions from conducting tests that use ballistic missile technology but has defied international warnings and sanctions to pursue missile and nuclear programs.

"Our assessment is that the North will use the newly upgraded Tongchang-ri (missile) launch pad to launch a long-range missile larger than Unha-3," Yonhap quoted a South Korean government source as saying.

Unha-3 refers to the long-range rocket fired in 2012 from the base near its west coast.

The launch is likely to come around the 70th anniversary of the founding of the North's ruling Workers' Party on Oct. 10, the source was quoted as saying.

South Korea's Defense Ministry could not immediately confirm the report but an official said they were seeking to verify it.

North Korea is believed to be developing an intercontinental ballistic missile (ICBM) but is seen as years away from deployment. It is also working to miniaturize a nuclear warhead to mount on a delivery vehicle.