SEOUL, South Korea (AP) — North Korea said Monday it simulated a nuclear attack on South Korea with a ballistic missile launch over the weekend that was its fifth missile demonstration this month to protest the largest joint military exercises in years between the United States (US) and South Korea.

The North’s leader Kim Jong Un instructed his military to hold more drills to sharpen the war readiness of his nuclear forces in the face of “aggression” by his enemies, state media reported.

The South Korean and Japanese militaries detected the short-range missile being launched Sunday into waters off the North’s eastern coast, which reportedly came less than an hour before the US flew long-range B-1B bombers for training with South Korean warplanes. The North characterises the US-South Korea exercises as a rehearsal to invade, though the allies insist they are defensive in nature. Some experts say the North uses the exercises as a pretext to advance its weapons programs.

Pyongyang’s official Korean Central News Agency said the missile, which flew about 800 kilometres (500 miles), was tipped with a mock nuclear warhead. It described the test as successful, saying that the device detonated as intended 800 metres (yards) above water at a spot that simulated an unspecified “major enemy target,” supposedly reaffirming the reliability of the weapon’s nuclear explosion control devices and warhead detonators.

The report said the launch was the final step of a two-day drill that also involved nuclear command and control exercises and training military units to switch more quickly into nuclear counterattack posture, properly handle nuclear weapons systems and execute attack plans.

The exercise was also a “stronger warning” to the United States and South Korea, who are “undisguised in their explicit attempt to unleash a war” against the North, KCNA said.

Photos published by state media showed Kim walking through a forest with his daughter and senior military officials and a missile the North described as a tactical nuclear weapon system soaring from the woods spewing flames and smoke.

Saying that his enemies are getting “ever more pronounced in their moves for aggression,” Kim laid out unspecified “strategic tasks” for further developing his nuclear forces and improving their war readiness, KCNA said. This indicated that the North could up the ante in its weapons demonstrations in coming weeks or months.

Jeon Ha Gyu, spokesperson of South Korea’s Defense Ministry, said it’s clear North Korea with its ramped-up testing activity is making “considerable progress” in nuclear weapons technology. He did not provide a specific assessment about the North’s claim about the successful warhead detonation.

North Korean photos indicated the latest launch was of a solid-fuel missile apparently modelled after Russia’s Iskander mobile ballistic system that the North has been testing since 2019. The missiles are built to travel at low altitudes and be manoeuvrable in flight, which theoretically improve their chances of evading South Korean missile defences.

While these missiles have been mostly fired from wheeled vehicles, North Korea has also tested them or their variants from railcars, a submarine and a platform inside a reservoir. Photos of the latest test suggested the missile was possibly fired from a silo dug into the ground, highlighting the North’s efforts to diversify its launch options and make it harder for opponents to identify and counter them.

South Korea’s military said the launch took place at a mountainous northwestern region near Tongchang Ri, which hosts a site where the North conducted long-range rocket and satellite launches in previous years.