Pyongyang, North Korea - North Korea warned the US on Monday not to send a nuclear-powered submarine with ballistic missiles to South Korea.

The North Korean Defense Ministry said the US was planning to bring a strategic nuclear submarine with nuclear warheads to the Korean peninsula.

The move could "practically trigger the worst crisis of nuclear conflict," a spokesman was quoted as saying by state-controlled media. He accused the US of "blatant nuclear blackmail."

In light of increased tensions with North Korea, the US and its ally South Korea agreed in April to strengthen their military cooperation. This includes a more visible deployment of strategic US weapon systems, including nuclear submarines with ballistic missiles.

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Depending on their design, this type of missile can also be equipped with one or more nuclear warheads. In June, the US forces sent a nuclear submarine with cruise missiles to South Korea for joint exercises.

North Korea is subject to tough international sanctions because of its nuclear weapons program. After an unprecedented series of missile tests last year, the isolated country has again launched nuclear-capable missiles several times this year.

It also attempted and failed to launch a spy satellite, but examinations of the debris recovered by South Korea revealed it had no military potential.