"This public display of contempt for universal norms against chemical weapons use further demonstrates the reckless nature of North Korea and underscores that we cannot afford to tolerate a North Korean WMD (weapons of mass destruction) program of any kind", State Department spokeswoman Heather Nauert said in a written statement.
While in Pyongyang, Chung and a delegation he led had a four-hour dinner with Kim and his wife as well as other senior officials, including Kim's sister, Kim Yo Jong, who visited South Korea for the opening of the Winter Olympics last month.
Trump told reporters in the Oval Office the US had "come a long way, at least rhetorically" with North Korea and "statements coming out of South Korea and North Korea have been very positive".
North Korean leader Kim Jong-un may propose sending his sister, Kim Yo-jong, to the US as part of efforts to launch direct talks between Washington and Pyongyang, according to a South Korean diplomatic source. Of the possibility for peacefully resolving the nations' deep differences, he said: "It'd be a great thing for the world, would be a great for North Korea, it would be a great thing for the peninsula".
Moon's special envoy, Chung Eui-yong, will relay an "undisclosed message" from Kim to Trump when he arrives in Washington on Thursday. "So we'll see how it all comes about", the President said earlier.
Trump tweeted: "Possible progress being made in talks with North Korea. You bet I'm scared, but what can I do?" said Wang Li, a university bookstore clerk looking after her nephew at the store.
South Korean presidential envoys have returned home a day after they met North Korean leader Kim Jong Un in a rare visit to the North.
But then, of course, came the topic of Trump's Twitter response to all this, which came on Tuesday morning: "Possible progress being made in talks with North Korea".
The two leaders will hold talks at Panmunjom on the highly militarised border.
Choi estimates that North Korea's foreign currency reserves are only about $4 billion to $5 billion, compared the South's $395 billion.
Moon has expressed an interest in reopening the Kaesong Industrial Complex, an experiment blending South Korean management and capital with North Korean labor that opened just north of the Demilitarized Zone in 2004.
North Korea has indicated an interest in denuclearization to past presidents, but had reversed course and not followed through.
North Korea has boasted of its plans to develop a nuclear-tipped missile capable of reaching the mainland United States. Alternatively, Washington and Pyongyang achieve a deal, then the North Koreans appear to cheat. The rest were Yoo Seong-min of the minor opposition Bareunmirae Party, Cho Bae-sook of the Party for Democracy and Peace and Lee Jeong-mi of the progressive Justice Party.
But a China Daily editorial on Wednesday warned Pyongyang that it could not blame others if the concrete steps it had recently taken to improve inter-Korean ties were greeted with caution. As appalling as it is to acknowledge this, Kim's negotiating position is much stronger now. North Korea has also denied the allegations in the UN report.
In contrast to the hawks, another group of administration officials see North Korea's overture as a good sign, because it will reduce tensions and give diplomacy more time to work. "The end result of joint efforts along these lines should be a comprehensive settlement of the situation in this subregion and the creation of a lasting peace and security mechanism in Northeast Asia".
But until then, Trump voiced a tentative openness to North Korea's "very positive" offer, even if his administration retains deep doubts.
Gary Samore is the executive director for research, Belfer Center for Science and International Affairs, Harvard Kennedy School and former senior adviser on weapons of mass destruction to President Barack Obama.