The secretive state hasn’t exploded a weapon at its Punggye-ri Nuclear Test Site since September 2016.
But the site is closely watched via satellite for any evidence that another nuclear blast is imminent.
Now spy bosses in neighbouring South Korea have warned that a test may be just days away.
The bombs are tested underground, beneath Mount Mantap, so signs of excavation are closely watched for.
South Korea’s National Intelligence Service (NIS) now believes that preparations in two test tunnels are complete.
It’s believed that the upcoming test, revealed in a parliamentary briefing by NIS chief Suh Hoon, could fall on September 9.
That’s a national holiday in North Korea, the Day of the Foundation of the Republic, and the same date when Kim last conducted a nuclear test.
South Korea-based reporter Christine Kim, speaking from Seoul, said the North wanted to make its nuclear warheads smaller so they could be mounted on a missile.
She said: “Experts believe North Korea has secured some technology to make its warheads small but it’s not quite there yet so more testing is needed.
“In order for missiles to fly a long distance, anything attached… would have to be lightweight to ensure the missiles fly for longer and farther.”
It comes just days after North Korea fired three missiles towards Japan, with one exploding shortly after launch and two hitting the sea.
At the same time, the US and South Korea are conducting Ulchi-Freedom Guardian, a joint drill preparing for a Northern attack.
The drill made Kim so angry that he held his own exercise – a mock invasion of the South Korean islands of Baengnyeongdo and Big Yeonpyeong.