(Bloomberg) -- North Korea has breached the cap on fuel imports imposed by the United Nations Security Council, likely using illicit transfers between tankers at sea, according to the U.S. government.

The U.S. submitted a report to the UN that concludes North Korean ships have called in port at least 89 times between Jan. 1 and May 30 to deliver refined petroleum products procured via so-called ship-to-ship transfers, according to a copy of the report seen by Bloomberg News. Since the annual quota has been passed, the U.S. also demanded China and Russia stop selling the products to North Korea.

Ship-to-ship transfers, when cargoes are pumped from one tanker to another in the ocean, are legal and typically used to break up large oil shipments into parcels on smaller vessels. But they can also be used as a trick that makes it difficult to track supplies, and have been barred by the United Nations for sales to North Korea.

“We have information indicating this practice is occurring on a much greater scale than has been reported publicly,” the report said.

For more, read: North Korea Keeps Sneaking in Oil With Secret Ship Transfers

The report -- coming exactly one month after President Donald Trump’s summit with Kim Jong Un in Singapore -- is the latest sign of tension in nuclear talks between the two sides. After North Korea criticized U.S. negotiating tactics last week, Secretary of State Mike Pompeo said he wanted to maintain the “maximum pressure” campaign against the regime -- a term Trump has said he was avoiding in order to foster diplomacy.

The Security Council in December approved tighter sanctions on North Korea that included measures aimed at slashing deliveries of petroleum products to the equivalent of 500,000 barrels per year starting Jan. 1. The cap is among a raft of restrictions aimed at forcing Kim’s regime to abandon its nuclear weapons program.

The U.S. report determined import volumes have breached sanctions by estimating how full the ships were when they arrived at North Korean ports, including Nampo, Najin, Wonsan, Chongjin and Hamhung. At 33 percent full, or laden, the total would be about 507,000 barrels. The volume jumps to 760,000 barrels assuming the ships were half full and to almost 1.4 million barrels assuming they were 90 percent laden.

--With assistance from Nick Wadhams.

To contact the reporter on this story: Ramsey Al-Rikabi in Singapore at ralrikabi@bloomberg.net

To contact the editors responsible for this story: Bill Faries at wfaries@bloomberg.net, Pratish Narayanan

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