The United States deployed supersonic bombers and fighter jets after recent missile tests carried out by North Korea.
The drills demonstrated an “iron-clad” US security commitment to South Korea, they added.
It came after North Korea sparked widespread condemnation after testing the Hwasong-17 missile – their longest-range weapon, which is reportedly capable of hitting the American mainland - and warned Washington of “fiercer military responses”.
Attending the site with his daughter for the first time, Kim Jong-un said threats from the US prompted his country to “substantially accelerate the bolstering of its overwhelming nuclear deterrence.”
“Kim Jong-un solemnly declared that if the enemies continue to pose threats our party and government will resolutely react to nukes with nuclear weapons and to total confrontation with all-out confrontation,” the official KCNA news agency said.
The missile reportedly flew nearly 1,000 km (621 miles) for about 69 minutes and reached a maximum altitude of 6,041 km.
The launch on Friday was the latest in an unusually intense campaign of test firings by the North Korean regime, amid warnings from the South and the US that Pyongyang may be preparing for a nuclear test.
In recent months, North Korea has performed dozens of shorter-range missile tests that it called simulations of nuclear attacks on South Korean and US targets.
North Korea said its tests were aimed at issuing a warning to the United States and South Korea over their military training that the North views as an invasion rehearsal.
Before the launch on Friday, North Korean Foreign Minister Choe Son Hui threatened to launch "fiercer" military responses to steps by the U.S. to bolster its security commitment to South Korea and Japan.
US President Joe Biden met with his South Korean and Japanese counterparts on the side-lines of a regional gathering in Cambodia, issuing a joint statement that strongly condemned North Korea’s recent missile tests and agreed to work together to strengthen deterrence.
Mr Biden reaffirmed the US commitment to defend South Korea and Japan with a full range of capabilities, including nuclear weapons.