Of the four ships blocked from international ports under UN sanctions on North Korea, three are registered in the isolated country and a fourth flies the flag of Palau, according to the final list adopted by the world body.

Diplomats had said on Thursday that all four were North Korean vessels.

Blocking suspected vessels from ports -- except in the case of humanitarian need -- is provided for under UN Security Council sanctions resolutions targeting North Korea over its nuclear and missile programs.

According to the list seen by AFP, the ships in questions are the Palau-flagged oil tanker Billions 18, and the North Korean vessels Ul Ji Bong 6, Rung Ra 2 and Rye Song Gang 1.

The North Korean oil tanker Sam Jong 2 so far has not been banned from international ports, but it appears on a list of six ships suspected of transporting cargo banned under the international sanctions.

One diplomat said it was still under investigation.

The United States had originally asked for 10 ships to be banned, but China would only agree to four, diplomats said.

On Thursday, US President Donald Trump criticized China for failing to cut off Pyongyang's oil supply.

A State Department official said Washington was aware that "certain vessels have engaged in UN-prohibited activities, including ship-to-ship transfers of refined petroleum and the transport of coal from North Korea."

China, Pyongyang's main ally, has denied any wrongdoing.

So far, the UN has banned a total of eight ships from international ports.