Mr Kerry blamed North Korea for continuing to break promises, make threats and “show flagrant disregard for international law” by continuing provocative nuclear and missile activity while oppressing its own people.
He said North Korea’s “horrific conduct” must be exposed and vowed to ratchet up pressure on Pyongyang to change, particularly since it has rebuffed repeated attempts to restart de-nuclearisation negotiations.
“They have grown the threat of their programme and have acted with a kind of reckless abandon,” he said.
Mr Kerry’s accusations come less than a week after South Korea’s spy agency said North Korean leader, Kim Jong-un, ordered his defence chief executed with an anti-aircraft gun for complaining about the young ruler, talking back to him and sleeping during a meeting.
That allegation, if true, adds to concerns about the erratic nature of Mr Kim’s rule, particularly after Pyongyang claimed last weekend it had successfully test-fired a newly developed ballistic missile from a submarine.
Mr Kerry called the reported killing just the latest in a series of “grotesque, grisly, horrendous, public displays of executions on a whim and fancy”.
He said that if such behaviour continued, calls would grow for North Korea to be referred to the International Criminal Court.
Mr Kerry also expressed hope that the successful conclusion of a nuclear deal with Iran would send a positive message to North Korea to restart negotiations on its own atomic programme. He said he believed an Iran agreement could have “a positive influence” on North Korea, because it would show that giving up nuclear weapons improves economies and ends isolation.
The secretary of state added: “Perhaps that can serve as an example to North Korea about a better way to move, a better way to try and behave.”
International negotiators are rushing to finalise a nuclear deal with Iran by the end of June under which its programme would be curbed to prevent it from developing atomic weapons, in exchange for the lifting of international sanctions that have crippled its economy.
Nuclear talks with North Korea broke down three years ago. The secretive nation conducted its first nuclear test in 2006 and is now believed to have at least ten such weapons. It conducted its third nuclear test in February 2013 and US-based experts forecast it could increase its nuclear arsenal to between 20 and 100 weapons by 2020.
In addition to talks on North Korea, Mr Kerry is in Seoul to lay the groundwork for a visit to Washington in June by South Korean president Park Geun-hye.