The report by an independent panel of experts monitoring UN sanctions said the countries, which it did not identify, believed North Korea's past six nuclear tests had likely helped it develop miniaturised nuclear devices.
Pyongyang has not conducted a nuclear test since September 2017.
The interim report was submitted to the 15-member UN Security Council North Korea sanctions committee yesterday.
"The Democratic People's Republic of Korea is continuing its nuclear programme, including the production of highly enriched uranium and construction of an experimental light water reactor. A Member State assessed that the Democratic People's Republic of Korea is continuing production of nuclear weapons," it said.
North Korean leader Kim Jong Un said last week there would be no more war as the country's nuclear weapons guarantee its safety and future despite unabated outside pressure and military threats.