The United States said Thursday that North Korea has already exceeded the annual cap allowed for oil imports, and asked the United Nations to order an immediate halt to all transfers of oil to the country.

The request underscores the U.S. determination to keep squeezing North Korea through existing economic sanctions to get Pyongyang to give up its nuclear weapons that threaten the United States. Although tensions have eased up since President Trump met North Korean leader Kim Jong Un in Singapore last month, talks on denuclearization have hit a rocky spot since then.

North Korea characterized U.S. attitudes as being “gangster-like” when Secretary of State Mike Pompeo visited Pyongyang a week ago, and suggested it might not be willing to denuclearize after all. Sanctions adherence has slipped as trade has increased along China’s border with North Korea, which has been evading sanctions through dozens of open-sea oil transfers that are prohibited under U.N. sanctions.

In Thursday’s report submitted to the U.N. committee responsible for monitoring North Korean sanctions, the United States said North Korean tankers filled up with oil using ship-to-ship transfers at least 89 times in the first five months of 2018.

According to a copy of the report seen by The Washington Post, U.S. officials calculated that even if the tankers carried only one-third of their full load, North Korea would have already exceeded the annual quota of 500,000 barrels allowed under U.N. sanctions.

In a letter accompanying the report, the United States asks the sanctions committee to notify every country in the United Nations that North Korea has breached the annual quota. It also asks the committee to call for “enhanced vigilance” against attempts to circumvent sanctions through ship-to-ship transfers of oil.

The report includes a lengthy list of North Korean tanker deliveries this year, running two and a half pages. It suggests that North Korea has found ways to continue importing oil, even if the transactions keep getting captured on U.S. and Japanese intelligence photographs.

U.S. officials have insisted that Washington’s policy of imposing sanctions on North Korea remains as tough as ever. But last month, Trump said he did not want to use the phrase “maximum pressure” any longer to describe the sanctions campaign, because the two sides were “getting along.” He also said the administration would hold off imposing more sanctions for the time being.

North Korea already is under some of the strictest sanctions ever imposed on a country.

In April, the Security Council blacklisted dozens of ships and shipping companies over coal and oil smuggling to North Korea. Five were based in China. And in February the Trump administration cited more than 50 ships and shipping companies that it said had been evading sanctions, including ships flying flags of China, Singapore, Taiwan, Panama, Tanzania, the Marshall Islands and the Comoros.

The U.S. report notes that Russia and China have reported to the sanctions committee that they have sold petroleum to North Korea, as is permitted under the 500,000 barrel cap. The letter said that those sales must stop immediately.

It lists dozens of cases involving North Korean-flagged ships delivering oil to ports in their home country. But it includes only one specific instance of a Russian-flagged tanker, the Patriot, conducting a ship-to-ship transfer, on April 10, with the North Korean tanker Wan Heng 11. The North Korean tanker then proceeded to the port of Nampo in North Korea, docking five days later.

The naming of Russia comes four days before Trump is due to meet with Russian President Vladi­mir Putin in Helsinki.

It also comes on a day when North Korean officials did not show up for an expected meeting with U.S. officials in the demilitarized zone to discuss the repatriation of the remains of U.S. soldiers dating from the Korean War.