North Korean state media on Friday accused the U.S. and South Korea of conspiring to assassinate Kim Jong Un using a “biochemical” substance.

As tensions remain high over North Korea's expanding weapons program and missile tests, news agency KCNA reportedly said the CIA and South Korea’s intelligence agency backed a terrorist group that entered the country to kill Kim with “biochemical substances including radioactive substance and nano poisonous substance.”

North Korea vowed to “mercilessly destroy” the terrorists, the BBC reported Friday morning.

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The North has a history of making unfounded claims and is known for decrying international provocation on its own terms. But South Korea has in the past admitted to having plans in place to kill Kim.

The accusation comes just months after the assassination of half-brother Kim Jong Nam via a nerve agent in Kuala Lumpur. Kim's government is suspected in the slaying but has denied any involvement.

In recent weeks, the Trump administration and Asian allies to the U.S. have ramped up military pressure on North Korea, which has remained defiant with missile tests, military parades and other saber-rattling gestures.

During a recent trip to the Asia Pacific region, Vice President Pence said the “era of strategic patience” with North Korea is over.