She went on to say that "the United States strongly condemns the use of chemical weapons to conduct an assassination".
"We hope that all relevant parties can seize the current opportunity, work for the shared goal and make concerted efforts to promote the process of denuclearisation of the Peninsula and politically resolving the Korean Peninsula issue", Mr Geng said.
The caution extended by the U.S.is understandable because talks with North Korea over the past three decades have always veered off course and failed.
"This is the sort of rhetoric that one encounters at the bare beginnings of a process that can last years, and of course the only processes that have been run with North Korea on the nuclear issue have come a cropper", said a former USA official involved in talks with North Korea who spoke on condition of anonymity, using an idiom meaning failed.
On-again off-again "six-party" talks, grouping the two Koreas, Japan, Russia and the United States and hosted by China, ended in failure in 2009, with North Korea criticising US aggression.
South Korea also said North Korea has agreed to halt tests of nuclear weapons and missiles if it holds talks with the United States on denuclearization.
According to the State Department, the use of the chemical agent VX at the Kuala Lumpur airport was sanctioned by North Korea.
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Pyongyang was also said to be ready to halt nuclear and missile tests, and consider the dramatic step of abandoning costly and controversial WMD programmes if the United States agrees not to attack or overthrow the regime.
Last week, Trump responded to an overture from North Korea for talks with the U.S.by saying that will happen only "under the right conditions".
"I think they're honest also because of the sanctions and what we're doing with respect to North Korea", Trump said. Pyongyang will also suspend nuclear tests for as long as the talks are under way.
The chief presidential envoy said Pyongyang requested Seoul to send a taekwondo demonstration team and an art troupe to the North, in a bid to sustain the reconciliatory momentum between the two Koreas.
After returning from the United States, Chung will visit China and Russian Federation, while Suh Hoon, the head of South Korea's intelligence agency, will head to Japan to brief officials.
In Chung's account, Kim indicated he would not need to keep nuclear weapons if military threats against North Korea were removed and his nation received a credible security guarantee.
Moon's office is expected to hold a media briefing on the outcome of the visit later Tuesday. Inter-Korean talks began after Kim Jong Un said in his New Year's address that he wanted to engage the South.
The rival Koreas have been taking steps to fix ties strained by North Korean nuclear weapons and missile tests after the North reached out to the South over the recently concluded Pyeongchang Olympics.