Former US general Jack Keane said it was “remarkable” how much North Korea has been able to advance its nuclear programme in the last few years.
And the former general suggested Kim Jong-un could have turned to China or Russia to help boost his nuclear arsenal.
Mr Keane told Fox News: “I think they must’ve got some help from Chinese or Russian engineers or scientists because they had a miserable track record before the last few years.”
North Korea’s most recent missile test took place on November 29, and the regime said the Hwasong-15 missile flew 590 miles during its 53-minute flight before landing in the sea near Japan.
He said: “North Korea has always used negotiations in the past to buy some time to advance their programme and also to promote some goodwill for themselves.”
As a result, Mr Keane also warned Donald Trump must prepare for a nuclear “showdown” with North Korea in a matter of months.
But he said North Korea has finally come to the realisation that the US President and his team have “truly put the military option on the table”.
North Korea claimed it was a “pipe dream” to think it would ever give up its nuclear programme after the UN imposed brutal sanctions to halt Kim Jong-un’s regime.
In an effort to stop the rogue nation, the United Nations Security Council imposed tough sanctions on North Korea after its continued missile testing.
The resolution seeks to ban nearly 90 percent of refined petroleum product exports to North Korea.
China’s exports to North Korea increased more than 20 percent in the first three quarters of 2017, according to customs data.
But UN sanctions imposed in November forced China to stop all exports of petroleum to North Korea.
Chinese Foreign Ministry spokeswoman Hua Chunying said: “As a principle, China has consistently fully, correctly, conscientiously and strictly enforced relevant UN Security Council resolutions on North Korea.
“We have already established a set of effective operating mechanisms and methods.”