A spokesperson for the North's Ministry of National Defence said the United States has "intensified espionage activities beyond the wartime level", with "provocative" flights made by US spy aircraft over eight straight days this month, and one reconnaissance plane intruding into its airspace over the East Sea "several times".

"There is no guarantee that such shocking accident as downing of the US Air Force strategic reconnaissance plane will not happen in the East Sea of Korea," the spokesperson said in a statement, carried by the official Korean Central News Agency.

The spokesperson cited past incidents when Pyongyang shot down US aircraft, and warned the United States would pay for its "frantically staged" air espionage.

Late on Monday, North Korean leader Kim Jong Un's powerful sister Kim Yo Jong said that a US spy aircraft had violated the country's eastern airspace twice on Monday morning, according to a statement.

Kim Yo Jong said that the North would not respond directly to US reconnaissance activities outside of the country's exclusive economic zone, but warned that it would take "decisive action" if the US military crosses its maritime military demarcation line.

The earlier KCNA statement also slammed the planned deployment of US strategic nuclear assets to the Korean peninsula as "the most undisguised nuclear blackmail" against North Korea, saying it posed a grave threat to regional and global security.

"The present situation clearly proves that the situation of the Korean peninsula is coming closer to the threshold of nuclear conflict due to the US provocative military action," it read.

Washington said in April that it would send a nuclear-armed ballistic submarine to make the first visit to a South Korean port in decades, without specifying the exact timing.

North Korea has conducted multiple sanctions-defying launches this year, including test-firing its most powerful intercontinental ballistic missiles, and in May attempting to put a military spy satellite into orbit.

South Korea's President Yoon Suk Yeol has ramped up defence cooperation with Washington in response, staging joint military exercises with advanced stealth jets and powerful US strategic assets.

Yoon is set to attend a NATO summit in Lithuania this week, seeking stronger cooperation with NATO members over North Korea's growing nuclear and missile threats, his office said.