U.S. Special Representative for North Korea, Sung Kim and Noh Kyu-duk, South Korea's Special Representative for Korean Peninsula Peace and Security Affairs, attend a briefing after their meeting at a hotel in Seoul, South Korea October 24, 2021. Ahn Youn
U.S. Special Representative for North Korea, Sung Kim and Noh Kyu-duk, South Korea's Special Representative for Korean Peninsula Peace and Security Affairs, attend a briefing after their meeting at a hotel in Seoul, South Korea October 24, 2021. Ahn Young-joon/Pool via REUTERS. Reuters / POOL

The United States and South Korea would maintain the "strongest possible joint deterrent" over North Korea's "escalatory actions", the U.S. envoy on North Korea said on Monday, amid concerns that Pyongyang was preparing to resume nuclear testing.

U.S. Special Representative Sung Kim and his deputy, Jung Pak, met South Korean officials, including nuclear envoy Noh Kyu-duk, after arriving in Seoul early on Monday for a five-day visit.

"It is extremely important for the United Nations Security Council to send a clear signal to the DPRK that we will not accept its escalatory tests as normal," Kim told reporters after his talks with Noh.

Kim was referring to North Korea by its official name, the Democratic People's Republic of Korea.

"We agreed on the need to maintain the strongest possible joint deterrent capability on the peninsula," he said.

Kim also said the allies would "respond responsibly and decisively to provocative behaviour," while underlining his willingness to engage with North Korea "anywhere without any conditions."

Kim's arrival coincided with the start of a nine-day annual joint military drill by U.S. and South Korean troops.

The exercise consists of "defensive command post training using computer simulation" and will not involve field manoeuvres by troops, South Korea's Joint Chiefs of Staff said on Sunday.

North Korea has condemned the joint drills as rehearsals for war, and they have been scaled back in recent years amid efforts to engage Pyongyang in diplomacy, and because of COVID-19 restrictions.

On Saturday, North Korea test fired what state media said were missiles involved in delivering tactical nuclear weapons.

The U.S. envoy has repeatedly offered to re-engage with North Korea, but Pyongyang has so far rebuffed those overtures, accusing Washington of maintaining hostile policies such as sanctions and the military drills.

Kim was also expected to meet with the transition team for President-elect Yoon Suk-yeol, who takes office in May.

A spokesperson for the team said there was no meeting confirmed between Yoon and Kim, but Yoon's foreign minister nominee, Park Jin, said he planned to meet Kim.

Kim also said at his talks with Noh that Washington looks forward to working closely with Yoon's team.