"Kim Jong-nam was assassinated presumably by or at the behest of North Korean authorities by means of a chemical weapon that is banned by international convention, and under that circumstance, I think all of us got that the notion of some North Korean diplomats prancing into the United States was unseemly and untimely," Russel said in his farewell briefing to reporters from Asian media.

Russel said the planned unofficial talks with the North Koreans were "sideswiped" by the killing.

"A North Korea that will commit murder by a WMD, that will threaten the use of nuclear weapons against the United States and its neighbors, and that will launch ballistic missiles repeatedly in the direction of Japan in contravention of its legal obligations under UN Security Council resolutions, is not going to look like a country that is serious about negotiations," Russel said.

Still, he said the decision to deny visas doesn't mean the US isn't interested in talks.

Russel, who is set to step down and move to the Asia Society think tank, rejected as "unwarranted" China's concern about the deployment of the US THAAD missile defense system in South Korea, saying the system won't compromise the security interests of any country.

He said the only driver for the deployment is North Korea's threats.

"We fully expect China to always act to safeguard its security interest, but China better give us and give the Republic of Korea the same right. No one is threatening China with nuclear armed ballistic missiles. Somebody is threatening the United States and threatening Japan and threatening the Republic of Korea with nuclear armed missiles. That's the threat we're responding to," he said.

Referring to a series of retaliatory measures China has been taking against South Korean firms, Russel urged China to "devote the energy and leverage that it is putting into economic pressure against the Republic of Korea" to persuading the North out of the current path, he said.

When it comes to Korea policy, Russel also said that the administration of US President Donald Trump, just like its predecessors, will always be guided by the objective of achieving the peaceful denuclearization of the Korean Peninsula."

The US will also be focused on deterring and defending against the threat of a nuclear attack or a weapons of mass destruction attack while making clear that a negotiated resolution is preferable to the alternatives, he said. (Yonhap)