SEOUL - South Korean President Yoon Suk-yeol and United States Vice-President Kamala Harris have reaffirmed their country's security alliance while jointly condemning North Korea's "provocative nuclear rhetoric" and the missile tests it conducted as the two allies resumed a naval drill.

The two sides also discussed their joint response to possible future provocations, which could include trilateral cooperation with Japan, the White House said.

Vice-President Harris iterated the US' commitment to defending South Korea and strengthening both nations' "combined defence posture", while underscoring that North Korea's actions "threaten peace and security in the Indo-Pacific region and the entire international community".

Pyongyang has carried out at least 21 rounds of missile launches since the beginning of the year. The latest was on Thursday - hours after Ms Harris wrapped up a one-day visit to Seoul.

The regime also conducted a missile test on Wednesday and on Sunday - the eve of a four-day naval drill by the US and South Korea that was held for the first time in five years.

Drills were suspended as Washington and Pyongyang embarked on denuclearisation talks from 2018, but resumed under the new Yoon administration that seeks to bolster South Korea's alliance with the US and toughen its stance against the North.

A senior US official had also warned that Pyongyang may conduct a nuclear test when Ms Harris is in the region. Reports said the regime has completed preparations for what would be its seventh nuclear test since 2006.

Ms Harris arrived in Seoul on Thursday morning, flying in from Tokyo where she attended the state funeral of former Japanese prime minister Shinzo Abe on Tuesday.

In talks with Mr Yoon at his office that lasted 85 minutes, the two sides discussed issues such as upholding the international rules-based order, preserving peace and stability across the Taiwan Strait, and the growing economic and technology partnership between US and South Korea.

Ms Harris also underscored the benefits of improving Japan-Korea bilateral ties.

She also emphasised the importance of US-Japan-South Korea trilateral cooperation and "welcomed additional initiatives on this front given our shared goals and security concerns", the White House said.

Mr Yoon also raised concerns over America's new Inflation Reduction Act (IRA), which excludes South Korean businesses from US government subsidies, most notably for electric vehicles assembled outside of the US.

The President voiced "hopes that the two countries will work closely together to reach a mutually satisfactory agreement based on the spirit of the Korea-US free trade agreement", according to his spokesman.

Ms Harris said she understood South Korea's concerns, and both leaders "pledged to continue to consult as the law is implemented", according to the White House.

She also responded to the media controversy over Mr Yoon's swearing gaffe in New York last week, whereby he was caught on camera commenting to one of his aides about "losing face if these f****** do not pass it" in Congress.

International reports swirled with postulations that he had been referring to US President Joe Biden and the IRA, although Mr Yoon claimed he was misquoted.

The main opposition Democratic Party is demanding the resignation of Foreign Minister Park Jin to take responsibility for what it calls a "diplomatic disaster".

Ms Harris said Washington is not bothered by it at all, said Mr Yoon's spokesman.

"(She said) President Biden has deep trust in President Yoon and is satisfied with his meetings with President Yoon," the spokesman added.

The two leaders agreed to make plans for Mr Yoon to visit the US next year to mark the 70th anniversary of their nations' alliance.

While in town, Ms Harris also visited the heavily guarded Demilitarised Zone that divides the two Koreas.

"In the South, we see a thriving democracy. In the North, we see a brutal dictatorship, rampant human rights violations and an unlawful weapons program that threatens peace and stability," she was cited as saying.

"The United States and the world seek a stable and peaceful Korean peninsula where the DPRK is no longer a threat," she added, referring to North Korea's official name, the Democratic People's Republic of Korea.