N.Korea nuclear strike only if survival threatened: US

"Although we assess with low confidence that the North would only attempt to use nuclear weapons against US forces or allies to preserve the Kim (Jong-Un) regime, we do not know what would constitute, from the North's perspective, crossing that threshold," James Clapper, director of national intelligence, said in an annual report to Congress on global security threats.

But the United States faces a challenge trying to discern North Korea's strategic calculations when it comes to its nuclear weapons.

"We do not know Pyongyang's nuclear doctrine or employment concepts," the report said.

North Korea leader Kim Jong-Un has threatened to "wipe out" a South Korean island amid fresh international pressure and new sanctions over the North's nuclear weapons and missile tests.

While much of the North's rhetoric has been dismissed as bluster, the latest threat to the border island of Baengnyeong, which has around 5,000 civilian residents, appears credible, analysts say.