South Korea's president Yoon Suk-yeol on Thursday ordered an update of the military's operational plans to address North Korea's growing nuclear and missile threats, his office said.

Yoon gave the instructions at his first visit to a military bunker in the capital Seoul that would serve as a command post the event of a war.

His visit coincided with the start on Monday of military drills by the armed forces of South Korea and the United States that are the largest in years.

The annual summertime exercises have been renamed Ulchi Freedom Shield and due to finish on Sept. 1. They involve the first field training between the two militaries since 2017 after being scaled back amid the COVID-19 pandemic and under Yoon's predecessor who sought to improve relations with North Korea. [nL1N2ZY03S]

Yoon highlighted that this year's drills were conducted under a changed scenario and the operational plans reflect North Korea's evolving threats.

"We need to urgently prepare measures to guarantee the lives and property of our people, including updating the operational plans against North Korea's nuclear and missile threats that are becoming a reality," Yoon told military commanders during the visit.

North Korea has conducted missile tests at an unprecedented pace this year and is ready to conduct its first nuclear test since 2017 at any time, Seoul officials said this week.

The isolated, nuclear-armed North fired two cruise missiles from the west coast last week, after South Korea and the United States began preliminary training for the drills.

Pyongyang has long criticised the combined exercises as "hostile policy" and a rehearsal for invasion.

Yoon, who has vowed to boost drills and overall readiness against the North, called for beefing up the military's independent capability to counter North Korean missiles, while reinforcing the extended U.S. deterrence including its nuclear umbrella.

He also ordered the commanders to speed up plans to set up the so-called "Kill Chain" system, designed to launch preemptive strikes against the North's missiles and possibly its senior leadership if an imminent attack is detected.