North Korea has developed the ability to miniaturise nuclear warheads which can fit inside missiles, US intelligence believes.
The secretive state could therefore have taken a crucial step towards becoming a fully-fledged nuclear power.
US intelligence officials made the assessment in a report published last month, according to the Washington Post, citing unnamed sources.
The revelation comes just weeks after North Korea successfully fired its first intercontinental ballistic missile – claiming it could strike the US.
Experts fear Pyongyang is quickly moving closer towards becoming a nuclear threat to countries around the world.
It comes as tensions between North Korea and the US continue to increase, with Kim Jong-un threatening “thousands-fold” revenge against the US after the UN passed a new set of stringent sanctions against the one-party state.
The Post said that the latest report was compiled by the US Defence Intelligence Agency last month.
Officials at the agency would not comment on Tuesday.
Meanwhile, Japan’s defence ministry has also assessed that it is possible North Korea has achieved the miniaturisation of nuclear weapons and has developed nuclear warheads.
While it is more than a decade since North Korea tested its first nuclear detonations, experts hoped it would be years before the country could fit nuclear warheads into missiles.
This threshold appears to have been passed, the report suggests.
In response to North Korea’s firing of an ICBM, the United Nations Security Council passed a stringent set of sanctions on Saturday aimed at curbing the country’s nuclear abilities.
These included the banning of £770 million worth of exports out of North Korea.
Donald Trump and South Korean president Moon Jae-in praised the move and reaffirmed in a phone conversation on Sunday that North Korea posed a “grave and growing” to countries around the world.
The sanctions passed by 15 votes to 0, including the support of China, North Korea’s only ally, which has urged the country to stop missile testing.
Pyongyang state media described the move as a “heinous US plot to isolate and stifle” the country.