U.S. Secretary of State John Kerry on Monday accused North Korea of a litany of crimes and atrocities while reassuring South Korea of America's "ironclad" security commitments.

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 its behavior, particularly since it has rebuffed repeated attempts to restart denuclearization negotiations.

"They have grown the threat of their program and have acted with a kind of reckless abandon," Kerry said, referring to North Korea and its leadership, less than a week after South Korea's spy agency said North Korean leader Kim Jong Un ordered his defense chief executed with an anti-aircraft gun for complaining about the young ruler, talking back to him and sleeping during a meeting Kim presided over.

That allegation, if true, adds to concerns about the erratic nature of Kim's rule, particularly after Pyongyang claimed last weekend it had successfully test-fired a newly developed ballistic missile from a submarine.

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 behavior continued, calls would grow in the international community for North Korea to be referred to the International Criminal Court.

South Korean Foreign Minister Yun Byung-se said "the severity of recent threats and provocations" made it essential to bolster the security relationship.

The actions come despite a recent U.S. diplomatic overture to North Korea to discuss resuming denuclearization talks that have been stalled for the past three years. The U.S. quietly proposed a meeting with North Korea in January, before the U.S. and South Korea began annual military exercises that North Korea regards as a provocation. The two sides, however, failed to agree on who could meet and where.

Kerry noted North Korea's refusal to return to the table, saying "all they are doing now is isolating themselves further and creating greater risks to the region and to their own country." He said the U.S. remained open to talks but only if "we .... have some indication from the leader of North Korea that they are serious about engaging on the subject of their nuclear program."

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 program. Kerry said he believed an Iran agreement could have "a positive influence" on North Korea, because it would show that giving up nuclear weapons improves domestic economies and ends isolation.

"Perhaps that can serve as an example to North Korea about a better way to move, a better way to try and behave," he said.

International negotiators are rushing to finalize a nuclear deal with Iran by the end of June under which Iran's program 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, which has already developed atomic weapons despite previous attempts to forestall it, broke down three years ago as it has continued atomic tests and other belligerent behavior, including ballistic missile launches.

North Korea conducted its first nuclear test in 2006 and is now believed to have at least 10 such weapons despite some of the toughest international sanctions in existence. It conducted its third nuclear test in February 2013, and U.S.-based experts forecast that it could increase its nuclear arsenal to between 20 and 100 weapons by 2020.

In addition to talks on issues related to North Korea, Kerry in Seoul will be laying the groundwork for a visit to Washington in June of South Korean President Park Geun-hye.

Kerry is to deliver a speech on cyber security and related issues. Both North Korea and China pose major cyber security challenges. South Korea has faced hacking attacks it has blamed on North Korea, and the United States accuses the North of being behind the massive attack on Sony Pictures last year that resulted in new U.S. sanctions.

Kerry will use the opportunity to lay out U.S. efforts to combat the threats and to stress the importance of a free and open Internet, according to U.S. officials.