Pyongyang, North Korea - North Korea on Monday threatened to shoot down any US spy planes violating its airspace, not long after condemning Washington's plans to deploy a nuclear missile submarine near the Korean peninsula.

A spokesperson for the North's Ministry of National Defense 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 US would pay for its "frantically staged" air espionage.

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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 the country's exclusive economic zone, but warned that it would take "decisive action" if the US military crosses its maritime military demarcation line.

In a separate statement carried by KCNA Tuesday, Kim Yo Jong warned that the US military would face "a very critical flight" in case of a new intrusion by a US aircraft.

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 and risked provoking a "nuclear conflict."

The US 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.