Imports of goods and commodities by road and freight have also restarted, though not at pre-pandemic levels, a panel monitoring the enforcement of sanctions on North Korea said in a report to the Security Council.
The "confidential" United Nations report, obtained by Reuters on Monday, notes that several unidentified countries had assessed that Pyongyang "probably developed miniaturised nuclear devices to fit into the warheads of its ballistic missiles".
Those are believed to fit into its ballistic missile warheads.
The report says North Korea is continuing to produce highly enriched uranium and construct an experimental light water reactor. "A Member State assessed that the Democratic People's Republic of Korea is continuing production of nuclear weapons", the report said.
North Korea is formally known as the Democratic People's Republic of Korea (DPRK). North Korea's mission to the United Nations in NY did not immediately respond to a request for comment on the U.N. report.
Last week, North Korean leader Kim Jong Un said there would be no more war as the country's nuclear weapons guarantee its safety and future despite unabated outside pressure and military threats - a comment not a lot of people in the worldwide community believed.
Long-range nuclear weapons of the kind developed by Russian Federation and the USA contain up to 10 warheads along with decoys and chaff to make it hard for missile defence systems to shoot them down before they land.
The intercontinental ballistic missile Hwasong-14 is seen in this undated photo released by North Korea's Korean Central News Agency (KCNA) in Pyongyang July 5, 2017. But they did not allow experts to witness the dismantlement of the site.
In 2018, Kim and President Donald Trump met for the first time in Singapore, raising the hopes that the country could be open to ending its nuclear threats in exchange for sanctions relief from the United States.
The U.N. report said that since there is no indication of a comprehensive demolition, one unnamed country had assessed that North Korea could rebuild and reinstall the infrastructure needed to support a nuclear test - within three months.
The U.N. experts also say North Korea is violating sanctions through quote "illicit maritime exports of coal", though this was temporarily suspended due to the global health crisis.
'The Panel continues to assess that virtual asset service providers and virtual assets will continue to remain lucrative targets for the Democratic People's Republic of Korea to generate revenue, as well as mining cryptocurrencies, ' the report said.