U.S. Secretary of State John Kerry said Monday North Korean leader Kim Jong-un is "questionable in terms of judgment" and called his nuclear pursuit "one of the most serious issues on the planet today."

Kerry made the remark at a press roundtable during a visit to the Laotian capital of Vientiane, talking about his upcoming trip to China aimed at discussing how to deal with North Korea in the wake of its fourth nuclear test, according to a State Department transcript.

"I look forward to having solid conversations -- serious conversations -- about one of the most serious issues on the planet today, which is a clearly reckless and dangerous, evolving security threat in the hands of somebody who is questionable in terms of judgment and has proven thus to China," Kerry said.

Kerry declined to be more specific about what he'll be talking about with the Chinese, saying those talks should be private and he doesn't want to predetermine the discussions.

"We need to have the talks. That's why I'm going there: to hear their point of view, to share thoughts, to share ideas about how we can proceed -- all of us together -- in a smart and thoughtful way that could get a result," Kerry said.

"And I'm not going to get into the what ifs or things we may or may not do one way or the other. I think that deserves to be talked with them privately and carefully," he said.

Kerry's visit to Beijing, set to begin on Wednesday, will be closely watched as a gauge of how strong a punishment China will mete out to the recalcitrant ally that China also considers a strategic buffer against the United States and its allies.

China has condemned the North's nuclear test, but has been lukewarm about calls for stern responses. Analysts have long said that Beijing fears pushing Pyongyang too hard could lead to its collapse, instability on its border and ultimately the emergence of a pro-U.S. nation next door.

Chinese cooperation is key to putting together any meaningful punishment as it is one of the five veto-holding permanent members of the U.N. Security Council and the main provider of food and fuel for the impoverished North.

On Sunday, a senior State Department official said that the U.S. wants China to cooperate not only in U.N. Security Council sanctions, but also take unilateral steps to punish Pyongyang "as North Korea's lifeline, as North Korea's patron."

The official also called for China to "cut off avenues of proliferation and retard North Korea's ability to gain the wherewithal to advance its nuclear and its missile programs,"

saying Pyongyang is still engaged in illicit and proliferation activities through means involving transiting China.

Kerry has made no secret of his conviction that there is "much more that China can do by way of applying leverage," the official said, adding that the top American diplomat will be looking for practical and effective steps on the part of the Chinese. (Yonhap)