US Secretary of State John Kerry discussed security issues with top South Korean officials today following a new ballistic missile test by North Korea and the reported execution of its defence chief.

Kerry sat down with President Park Geun-Hye in Seoul, where he was also due to deliver a policy speech on cyber-space, which has become another outlet for North Korean belligerence.

In an effort to revive long-stalled denuclearisation talks, Washington says it is open to preliminary discussions with Pyongyang. But North Korea has responded with some significant military muscle flexing.

Just over a week ago, the North said it had successfully test-fired a submarine-launched ballistic missile (SLBM) -- a technology that could eventually offer the nuclear-armed state a survivable second-strike capability.

UN sanctions ban Pyongyang from using ballistic missile technology.

While North Korean leader Kim Jong-Un hailed the acquisition of a "world-class strategic weapon", US defence officials sought to play down the test, saying Pyongyang was still in the earliest stages of developing an SLBM capability.

Nevertheless, during a two-day stop in Beijing before arriving in Seoul, Kerry slammed the North's provocative and "destabilising" behaviour as "unacceptable".

Internal stability in North Korea has also become a concern after South Korea's intelligence agency reported last week that Pyongyang's defence minister had been purged and most likely executed.

The agency said it had unverified reports that the execution had been carried out at close range with a high-calibre anti-aircraft gun.

Some observers said the execution, if confirmed, suggested Kim was still struggling to cement his absolute authority in a country that has been ruled by his family for seven decades.

During his talks with Park and later with Foreign Minister Yun Byung-Se, Kerry was expected to underline US commitment to its military alliance with South Korea, which hosts a permanent deployment of close to 30,000 US troops.