(CNN) The United States has asked the United Nations to ban 10 ships from entering the world's ports over alleged dealings with North Korea.

The move comes after the UN blacklisted four ships in October, including one that was caught smuggling 30,000 North Korean-made rocket-propelled grenades in 2016.

Giovanni Davoli, a spokesman for the Italian mission to the UN, confirmed the US request, which was first reported by Reuters.

Italy is currently serving as chair of the UN Security Council's North Korea Sanctions Committee, the body charged with measuring and enforcing the measures levied against the hermit nation in response to its missile and nuclear tests.

The US State Department and the US Mission to the United Nations didn't immediately respond to a request for comment.

UN Security Council resolutions passed this year stipulate "all Member States shall prohibit the entry into their ports of such designated vessels," save for some circumstances, including in emergencies or if they are granted humanitarian exceptions by the UN.

However, it's up to individual UN members to enforce the measures, which analysts say aren't always clear cut.

Banned ship at Chinese port

An example of the nebulous nature of the sanctions arose this weekend.

The Hao Fan 6 , one of the four vessels handed a global port ban in October, was moored beside a port in eastern China on Sunday, according to tracking data published on multiple maritime monitoring websites.

On Monday, a staffer who answered the phone at the Zhoushan Marine Affairs Bureau confirmed the Hao Fan 6's location near Lujiashi Island, one of the Zhoushan Islands. He declined to give his name and to answer further questions.

"If there were no emergency or no special UN approval then this would appear to be a sanctions violation by China irrespective of how brief the stay at port may have been," said Nicholas Tam, a lawyer at Ince & Co specializing in shipping and trade sanctions.

However, George Lopez, a former member of the UN Panel of Experts on North Korea, said that by mooring beside the dock rather than physically attaching to it, the Hao Fan 6 could be trying to adhere to the letter of the UN sanctions.

"The fact that it's moored is probably a consciously chosen, ambiguous area (to comply with) strict application of the Security Council's intentions," Lopez told CNN.

The Hao Fan 6 seen in South Korea in 2015.

When asked about the Hao Fan 6's location and whether it had violated sanctions, Hugh Griffiths, the coordinator of the UN Panel of Experts on North Korea, which reports to the UN Security Council's North Korea Sanctions Committee, told CNN he could not comment as the matter is under Panel investigation.

Hua Chunying, a spokeswoman for the Chinese Foreign Ministry, said she was not aware of the ship's location or mission when asked Monday about the Hao Fan 6.

"I can tell you very responsibly that China carefully implements all UN Security Council resolutions related to North Korea," she said.

China accounts for nearly 90% of all trade with North Korea, a crucial economic lifeline for leader Kim Jong Un. Critics have contend China does not use that financial leverage to its fullest extent.

As the US tries to blacklist more vessels with North Korea ties, Lopez, the former UN panel member, says the Hao Fan's appearance off the Chinese coast will test the UN's willingness and ability to enforce maritime sanctions.

"In a world where the US especially keeps asking, is China our partner in enforcing sanctions, this particular ship's reappearance poses a good test."

"How does it appear and disappear, and now that it's found and it's on a list, what should happen?"