North Korea said Friday its latest cruise missile launches this week were part of nuclear attack simulations that also involved a test of a purported underwater attack drone as leader Kim Jong Un vowed to make his rivals “plunge into despair.”

North Korea has stepped up its weapons demonstrations this month in a tit-for-tat response to the United States' expanding military exercises with ally South Korea aimed at countering the North's growing nuclear threat. The allies completed an 11-day exercise that included their biggest field training in years on Thursday, but North Korea is expected to continue its weapons tests as the United States reportedly plans to send an aircraft carrier in coming days for another round of joint drills with the South.

Pyongyang's official Korean Central News Agency said Kim supervised a three-day exercise through Thursday that simulated nuclear counterattacks against enemy naval assets and ports that involved detonations of mock nuclear warheads. KCNA said that the drills were aimed at alerting the United States and South Korea of a brewing “nuclear crisis” as they continue with their “intentional, persistent and provocative war drills” the North portrays as invasion rehearsals.

KCNA said the drills verified the operational reliability of an underwater nuclear attack drone the North has been developing since 2012. It said the drone after being deployed in waters off the North's eastern coast on Tuesday traveled in various patterns underwater for nearly 60 hours before successfully executing an underwater detonation of a test warhead at a spot simulating an enemy port.

North Korea also test-fired cruise missiles from nearby waters on Wednesday, in what was its sixth missile event this month. The North previously staged another nuclear attack simulation with a short-range ballistic missile on Sunday and flight-tested an intercontinental ballistic missile last week.?

KCNA said the four cruise missiles involved in Wednesday's tests were of two different types. It said the missiles flew for more than two hours drawing oval and figure-eight shape patterns over the sea while demonstrating an ability to strike targets 1,500 kilometers (932 miles) and 1,800 kilometers (1,118 miles) away. It said the missiles successfully set off detonations of mock nuclear warheads 600 meters (1,968 feet) above their targets, which supposedly verified the reliability of their nuclear explosion control devices and warhead detonators.

KCNA said Kim was satisfied with the three-day drills and further issued unspecified “militant tasks” to counter the “reckless military provocations” of his rivals, indicating that the North would further ramp up its military displays.?

Kim “expressed his will to make the U.S. imperialists and the (South) Korean puppet regime plunge into despair” with powerful demonstrations of his military nuclear program to make his rivals understand “they are bound to lose more than they get” with the expansion of their joint drills.?

Kim issued similar language Sunday as he supervised a test-firing of a short-range ballistic missile launched toward the sea from what was possibly a silo dug into the ground. The North's media said a mock nuclear warhead placed on the missile detonated 800 meters (2,624 feet) above water, an altitude that some experts say was aimed at maximizing damage.

It was the first time for North Korea to publicize such an altitude for detonating a nuclear weapon though it has previously claimed to have conducted simulated nuclear strikes on its rivals.

The North has fired over 20 ballistic and cruise missiles over 10 different launch events this year as it tries to diversify its delivery systems and display a dual ability to conduct nuclear strikes on both South Korea and the U.S. mainland. North Korea already is coming off a record year in testing activity, with more than 70 missiles fired in 2022, as Kim accelerated a campaign aimed at forcing the United States to accept the idea of the North as a nuclear power and negotiating badly needed sanctions relief from a position of strength.

South Korea and the United States have been responding by expanding their joint military exercises that had been downsized in previous years, first to support the former Trump administration's diplomacy with Pyongyang and then because of COVID-19. Seoul's Defense Ministry said this week that South Korea and the U.S. are planning to conduct a live-fire exercise that would be “unprecedented” in scale in June.