North Korean leader Kim Jong Un vowed Tuesday to conduct a nuclear warhead test and ballistic missile launches "in a short time," state media reported.
The goal would be "to further enhance the reliance of nuclear attack capability," he said, according to the Korean Central News Agency.
Kim reportedly made the remarks while overseeing a simulated test of missile re-entry technology. This technology is needed for missiles to re-enter the Earth's atmosphere intact.
"Seoul believes that the North has yet to secure such technology," said an unnamed South Korean official who asked to remain anonymous.
The North Korean claim comes as South Korean and US forces are carrying out annual joint military manouevres.
Pyongyang said last week it had succeeded in developing the technology to fit a nuclear warhead on a missile.
Seoul said it doubted that North Korea had the ability to miniaturize an atomic warhead.
The United Nations Security Council earlier this month imposed its strictest sanctions ever on North Korea, in response to the tests of a nuclear device in January and a long-range rocket in February.
North Korea has carried out nuclear tests in 2006, 2006, 2013 and in January 2016.