Leader Kim Jong Un "guided the test-fires", the official Korean Central News Agency said, of the "new-type tactical ballistic missile Hwasongpho-11-Da-4.5 and an improved strategic cruise missile".

Kim has staged dozens of launches this year, part of a testing spree that experts say could be linked to North Korea's alleged illicit supplying of weapons to ally Russia for use in Ukraine.

"The new-type tactical ballistic missile was tipped with a 4.5 tonnage super-large conventional warhead according to design," KCNA said.

The North "also conducted a test-fire of strategic cruise missile whose performance has been highly upgraded for its combat use," it added.

Seoul's military had described the Wednesday launches as a salvo of short-range ballistic missiles, which it said had flown about 400 kilometres east of the Korean peninsula.

The launch "was aimed at verifying the accuracy of the hit at a medium range of 320 km and the explosive power of the super-large warhead," the KCNA said.

Kim said such a test was necessary "for the state security environment" of the country.

"The military and political situation in the region... indicates that the work for bolstering up the military capability for self-defence should be the most important affair," he said, KCNA reported.

The North has denied any sanctions-busting weapons trade with Russia, but with diplomacy long-stalled, it declared South Korea its "principal enemy" this year and recently moved nuclear-capable weapons to border areas.

It has also recently bolstered military ties with Moscow, with President Vladimir Putin making a rare visit to Pyongyang in June, where he signed a mutual defence agreement with Kim.

Kim received Russian security chief Sergei Shoigu in Pyongyang last weekend, and sent the North's foreign minister to Russia earlier this week.

The North is also preparing for a parliamentary meeting in October that is expected to approve measures likely to escalate tensions with South Korea, including incorporating the hostile relationship between the two Koreas into its constitution.