Earlier, Spicer issued a statement congratulating Moon on his election victory, saying the US looks forward to working with him to continue to strengthen the alliance between the two countries.

"We congratulate President-elect Moon Jae-in and join the people of South Korea in celebrating their peaceful, democratic transition of power," White House press secretary Sean Spicer said in a statement, referring to the process of impeaching and ousting former President Park Geun-hye and electing a new president.

"We look forward to working with President-elect Moon to continue to strengthen the alliance between the United States and the Republic of Korea and to deepen the enduring friendship and partnership between our two countries," Spicer said.

An unidentified US official was quoted as telling Reuters that Moon's election is not expected to significantly change the Korea-US alliance, even though his favoring of a more conciliatory approach toward North Korea and questioning of the deployment of the THAAD missile defense system could add more volatility to the relations.

"It remains a concern that the left of center, left-wing party in South Korea is going to do well," the official was quoted as saying. "But they are going to have to do some coalition building, so I am not sure he's going be able to have an unadulterated anti-alliance, anti-THAAD stance."

Moon's election "introduces a level of volatility," but will not likely change the alliance "significantly," the official said.

The official also said it's premature to talk about the possibility of Moon asking for the removal of the THAAD system, but added that campaign positions could be moderated once his administration conducts a full appraisal of the security situation. (Yonhap)