The United States requested the most recent ban along with measures targeting ships registered in other countries, diplomats said on condition of anonymity.
The ban of the four vessels -- the Ul Ji Bong 6, Rung Ra 2, Sam Jong 2 and Rye Song Gang 1 -- brings the UN's total number of blocked ships to eight. The United States requested the most recent ban along with measures targeting ships registered in other countries, diplomats said on condition of anonymity.
But China only agreed to target the four ships flying North Korean flags, diplomats said, as part of international efforts to curb Pyongyang's missile and nuclear programs.
"Only four ships have been accepted" for the ban but "the procedure remains open" to include other vessels in the future, one diplomat said.
The list submitted by the US in December had also included ships flying flags from Belize, China, Hong Kong, Palau and Panama.
Shortly before the Council decision US President Donald Trump -- who has often hailed China's efforts to put pressure on North Korea -- sharply criticized Beijing for failing to cut off Pyongyang's oil supply.
"Caught RED HANDED - very disappointed that China is allowing oil to go into North Korea," Trump said on Twitter.
"There will never be a friendly solution to the North Korea problem if this continues to happen!"
It was not immediately clear if Trump was referring to the report or US intelligence in his tweet, or if he was accusing China -- the North's main ally -- of directly violating sanctions targeting Pyongyang.
In 2017, the Security Council has slapped three sets of sanctions on North Korea: one on August 5 targeting the iron, coal and fishing industries; another set on September 11 aimed at textiles and limiting oil supply; and the most recent on December 22 focused on refined petroleum products.
The US has denounced trafficking of banned goods that allows North Korea to stock up, particularly the transfer of cargo between different ships on the high seas.
Blocking suspected vessels from ports -- except in the case of humanitarian need as determined by the council's sanctions committee -- is provided for in the August resolution.
On October 5, the UN had already identified four ships "carrying prohibited goods," resulting in a ban on port access that was a "first in United Nations" history, according to Hugh Griffiths, part of a UN panel of experts monitoring the application of sanctions on Pyongyang.
Those four vessels were registered in the Comoros, Saint Kitts and Nevis, Cambodia and North Korea, and were targeted for the illegal transport of coal, iron and North Korean fish.
South Korean newspaper Chosun Ilbo, citing government sources in Seoul, reported earlier this week that US satellites had spotted Chinese ships selling oil to North Korean vessels dozens of times since October.
Washington remains convinced that only pressure from the government of Chinese President Xi Jinping will persuade Kim to back down and negotiate an end to the nuclear standoff.