US Secretary of State John Kerry discussed security issues with top South Korean officials on Monday 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.

- Elite hackers -

In Beijing, Kerry had voiced hopes that the successful conclusion of an atomic deal with Iran would encourage North Korea to resume six-party negotiations on its nuclear programme.

The talks, grouping North and South Korea, Japan, the United States, China and Russia, were aimed at getting North Korea to give up its nuclear weapons in return for economic and diplomatic benefits, and security guarantees.

But the six-party forum has not met since December 2008.

President Barack Obama's administration has been accused of neglecting North Korea's nuclear programme while focusing its attention on Iran.

A recent report by US researchers warned that North Korea appeared poised to expand that programme over the next five years and, in a worst-case scenario, could possess 100 atomic arms by 2020.

North Korea carried out nuclear tests in 2006, 2009 and 2013, and has an active ballistic missile development programme, although expert opinion is split on how much progress it has made.

Later Monday, Kerry was scheduled to lay out a comprehensive foreign policy speech on the use of cyber-space, including issues of cyber-security.

Seoul has blamed North Korean hackers for a series of devastating cyber-attacks on military institutions, banks, government agencies, TV broadcasters and media websites in recent years.

The United States also blamed the North for a cyber-attack on Sony Pictures over its North Korea-themed satire "The Interview" last year.

Pyongyang denied involvement in the hack of the entertainment company but furiously condemned the film, which features a fictional plot to assassinate Kim.

South Korea's defence ministry believes North Korea runs an elite cyber-warfare unit with up to 6,000 personnel, and regards its ability to launch hacking attacks as a major security threat.

Last month, President Park appointed an army general to the new post of national cyber-security tsar, specifically tasked with defending against North Korean hacking.