This file photo, taken Jan. 4, shows the U.S. U-2S reconnaissance aircraft flying over Osan Air Base in Pyeongtaek, 60 kilometers south of Seoul. Yonhap

North Korea denounced on Monday a plan by the United States to send a strategic nuclear submarine to South Korea, warning it "may incite the worst crisis of nuclear conflict in practice."

The U.S. had pledged to send a nuclear-capable ballistic missile submarine to South Korea in the Washington Declaration issued by President Yoon Suk Yeol and U.S. President Joe Biden during their summit in Washington in April to further enhance the "regular visibility" of strategic assets on the Korean Peninsula.

"The deployment of the U.S. strategic nuclear submarine carrying nuclear warheads in the Korean peninsula means that the U.S. strategic nuclear weapons will appear in the peninsula for the first time since 1981," a spokesperson of the North's defense ministry said in a statement carried by the Korean Central News Agency (KCNA).

"This is a very dangerous situation as it will bring the regional military tension to a more critical state and may incite the worst crisis of nuclear conflict in practice," the unnamed spokesperson said.

The spokesperson called the U.S. plan to send a strategic nuclear submarine to the peninsula "the most undisguised nuclear blackmail against" North Korea.

North Korea "should show in the clearest way how it will take counteraction, in order to prevent the U.S. from doing such reckless acts with ease," the spokesperson said.

Also, the spokesperson accused a U.S. spy aircraft of intruding in its airspace recently, threatening that there is no guarantee such aircraft will not be shot down.

"In particular, a strategic reconnaissance plane of the U.S. Air Force illegally intruded into the inviolable airspace of the DPRK over its East Sea tens of kilometers several times," the spokesperson said.

DPRK stands for North Korea's official name, the Democratic People's Republic of Korea.

"There is no guarantee that such shocking accident as downing of the U.S. Air Force strategic reconnaissance plane will not happen in the East Sea of Korea," the spokesperson said.

The official cited past incidents when the North shot down U.S. aircraft, including an EC-121 reconnaissance aircraft in 1969 and a military helicopter in 1994.

Later in the day, Kim Yo-jong, the powerful sister of North Korean leader Kim Jong-un, also issued a statement alleging that a U.S. spy aircraft intruded in the North's eastern airspace twice at around 5 a.m. and 8:50 a.m., respectively, on Monday.

In a statement carried by the KCNA, she said the U.S. is "engaging in a serious military provocation by conducting aerial surveillance."

Kim Yo-jong added that the North will not respond directly to the U.S. reconnaissance activities outside of the country's exclusive economic zone, but warned that it will take "decisive action" if the U.S. military again crosses its maritime military demarcation line.

Meanwhile, South Korea's military rejected the North's claim of its airspace being violated as "not true" and described flights by U.S. aerial surveillance assets around the peninsula as part of regular surveillance activities.

"(We) sternly urge an immediate halt to acts that create tension through such false claims," Joint Chiefs of Staff spokesperson Col. Lee Sung-jun told reporters. (Yonhap)