This ban became known after the meeting of the United Nations on Monday.
US President Donald Trump's administration is now going after North Korea's money as part of its campaign to rein in the hermit nation's weapons programs.
Griffiths identified the four ships as Petrel 8, Hao Fan 6, Tong San 2, and Jie Shun. "But it's a port ban", said Hugh Griffiths, coordinator of the UN Security Council Panel of Experts on North Korea sanctions, adding the ships were found "transporting prohibited goods". "These four vessels are prohibited all the ports (...) to have transported goods that are prohibited", he added, refusing to specify.
The port bans were enacted under UN Security Council Resolution 2375, which was passed last month after North Korea test-fired a missile that overflew Japan.
The Hao Fan 6 is registered in St. Kitts and Nevis, tiny islands in the Caribbean, while the Petrel 8 is a cargo ship registered in the Comoros islands, off the coast of East Africa; according to MarineTraffic.com. In September, textile products were also banned, work permits for North Korean citizens overseas and, in part, imports of petroleum products.
The ban entered into force on October 5 and does not imply freezing assets or a ban on movement.
North Korea's main economic partner China has signed up to the measures, along with Russian Federation.
The vessels are the first to be designated under stepped-up sanctions imposed on North Korea by the 15-member council in August and September over Pyongyang's sixth and largest nuclear test and two long-range ballistic missile launches. Both have previously vetoed harsher sanctions on the secretive state. In addition to lead, those goods are estimated to total more than $1 billion - about one-third of the country's estimated US$3 billion in exports in 2016. AFP quotes North Korea's ambassador to the United Nations, Ja Song-nam, vowing that North Korea will withstand all sanctions and continue down "the road of building socialist power by dint of the spirit of self-reliance and self-development".