The United States believes the ongoing deployment of the THAAD missile defense system has nothing to do with the political situation in South Korea because it's a "very utterly reasonable step" against North Korean threats, a senior State Department official said Monday.

Acting Assistant Secretary Susan Thornton made the remark during a Foreign Press Center briefing previewing Secretary of State Rex Tilleron's upcoming trip to Seoul, Tokyo and Beijing as South Korea prepares to elect a new leader after former President Park Geun-hye was ousted from office for corruption.

"I think we're certainly aware that there's going to be an upcoming political process and a set of new elections for president coming up and that there may be candidates that will be different from those that have been in the administration in recent years," Thornton said.

She said it's difficult to say what the situation means for THAAD because it's not clear even who the candidates are.

"But the way we look at the THAAD deployment is it's a very utterly reasonable step for a country to take, which sees itself threatened by missiles from its northern neighbor, and not only sees itself threatened, but these threats and these provocations are coming now with great regularity and it's very alarming to people who have to live under that kind of situation," Thornton said.

"So I think for us, the THAAD deployment is not related to some political constellation or other consideration," she said. "It's a very reasonable and real response to a very provocative threat that is facing people in South Korea."

Asked if the US would accept a reversal in South Korea's decision to host THAAD, Thornton said that the deployment was an alliance decision made jointly between the US and the South.

Park's ouster came just days after the US brought into South Korea the first elements of a Terminal High Altitude Area Defense battery that the two countries agreed to host in the country to defend better against North Korean missile threats.

Her exit could raise doubts about her decision to host THAAD amid intensifying pressure from China, which has strongly railed against the deployment, claiming the system, especially its powerful radar, can be used against the country.

Thornton also said that Park's impeachment and ultimate removal from office took place through "a pretty transparent process undergirded by varying democratic institutions which have really proven their mettle in the unfolding of this peaceful process."

She said that how to deal with North Korea will be an important topic in Tillerson's trip.

"This trip will allow Secretary Tillerson to engage allies and partners on not only a range of bilateral issues, also importantly to discuss and coordinate strategy to address the advancing nuclear missile threat from North Korea," Thornton said.

"The United States is committed to holding North Korean accountable for its flagrant and repeated disregard of UN Security Council resolutions which explicitly prohibit ballistic missile and nuclear programs. We will defend our friends and allies, of course, the Republic of Korea and Japan and seek to work collaboratively to the maximum extent possible with important partners like China on this issue," she said.

Tillerson is scheduled to leave Tuesday for the trip that will take him to Tokyo on Wednesday, Seoul on Friday and Beijing on Saturday. While in Seoul, Tillerson is scheduled to meet with Acting President and Prime Minister Hwang Kyo-ahn and Foreign Minister Yun Byung-se.

The trip comes as the administration of President Donald Trump is putting together its North Korea policy.

"The current administration, especially following on the missile launches that occurred a couple of weeks ago, is looking very actively at the issue of North Korea and reviewing its approach and what kinds of options it's considering. That process is still ongoing," Thornton said.

She said the US is "intent on keeping up the pressure on the North Korean regime and trying to change the calculus on the part of the North Korean leadership to come back to the negotiating table and discuss its illegal and very destabilizing weapons programs."

"Certainly discussing the elements of any new approach will be an important part of discussions," she said. (Yonhap)