Trump issued the warning during a briefing on opioid addiction at his golf course in Bedminster, New Jersey.
A Japanese defense paper and a U.S. media report said Tuesday that North Korea may have successfully produced a miniaturized nuclear warhead that can fit inside its missiles.
That's a key benchmark in the country's attempt to become a full-fledged nuclear power.
Washington's alarm over North Korean leader Kim Jong Un's pursuit of a nuclear capability has intensified in the past month after the North conducted two tests of intercontinental ballistic missiles last month.
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