U.S. President Donald Trump said on Tuesday North Korea "will be met with fire and fury like the world has never seen" if it threatened the United States again.
Earlier Pyongyang said it was ready to give Washington a "severe lesson" with its strategic nuclear force in response to any U.S. military action.
Washington has warned it is ready to use force if need be to stop North Korea's weapons ballistic missile and nuclear programs but has said it prefers global diplomatic action, including sanctions.
The consequences of a U.S. strike would potentially be catastrophic for South Koreans, Japanese and U.S. military personnel within range of North Korean retaliatory strikes.
"North Korea best not make any more threats to the United States. They will be met with fire and fury like the world has never seen," Trump told reporters at the Trump National Golf Club in Bedminster, New Jersey.
The president's remarks came a day after North Korea said it will launch "thousands-fold" revenge against the United States over the adoption of new, tough UN sanctions.
The United Nations Security Council unanimously approved on Saturday new sanctions to punish the North including a ban on coal and other exports worth over $1 billion, a third of North Korea's $3 billion annual earnings.
Earlier on Tuesday, The Washington Post reported that North Korea has developed miniaturized nuclear warheads that could be installed inside its missiles, taking an additional step towards becoming a nuclear power. The report was based on a new U.S. intelligence assessment.
The confidential conclusion by the Defense Intelligence Agency says the U.S. intelligence community has concluded "North Korea has produced nuclear weapons for ballistic missile delivery, to include delivery by [intercontinental ballistic misile]-class missiles," the Washington Post on Tuesday quoted the assessment as saying.
Japan also said on Tuesday that it was possible that North Korea had already developed nuclear warheads.
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