North Korea is getting around United Nations sanctions by transferring oil at sea, according to a new report from NBC.

The transfers are taking place despite U.S. efforts to stop them.

A top-secret U.S. military assessment found that warships and surveillance aircraft deployed by a U.S.-led, eight-nation coalition since September has forced Pyongyang to transfer oil farther away from the Korean Peninsula, NBC reported, citing three U.S. officials familiar with the intelligence.

The transfers are now taking place in the territorial waters of other countries, according to the U.S. Pacific Command report.

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The North Koreans are also using smaller ships to evade being spotted by the coalition's ships and aircraft, the officials told NBC.

While attempts to transfer oil have not decreased, the change in locations could eventually affect the pace and number of transfers, upping the cost of smuggling for North Korea, according to the officials.

The Trump administration has struggled to keep economic pressure on North Korea as it attempts to convince the isolated nation to give up its nuclear weapons. The nation's economy depends on the smuggled oil.

Following publication of NBC’s report, President Trump Donald John TrumpBiden on Trump's refusal to commit to peaceful transfer of power: 'What country are we in?' Romney: 'Unthinkable and unacceptable' to not commit to peaceful transition of power Two Louisville police officers shot amid Breonna Taylor grand jury protests MORE wrote on Twitter that the administration is “in no hurry” in its nuclear negotiations with Pyongyang.

“Many people have asked how we are doing in our negotiations with North Korea — I always reply by saying we are in no hurry, there is wonderful potential for great economic success for that country. Kim Jong Un sees it better than anyone and will fully take advantage of it for his people. We are doing just fine!”

Trump met with North Korean leader Kim Jong Un in June to discuss denuclearization of the Korean Peninsula, but negotiations have stalled in the six months since.

A meeting set for early November between Secretary of State Mike Pompeo Michael (Mike) Richard PompeoTreasury sanctions individuals, groups tied to Russian malign influence activities Navalny released from hospital after suspected poisoning Overnight Defense: Pentagon redirects pandemic funding to defense contractors | US planning for full Afghanistan withdrawal by May | Anti-Trump GOP group puts ads in military papers MORE and a top North Korean official was indefinitely postponed, though administration officials have said Trump expects to meet Kim for a second summit early next year.

North Korea has perfected ways to get around international prohibitions, using shell companies, illicit financing and illegal shipping, and by having China and Russia smuggle in prohibited items, experts and former officials told NBC.

Under a U.N. Security Council resolution in September 2017, there is a cap on refined imports at 500,000 barrels a year for North Korea.

To attempt to quell the oil smuggling, Washington in October 2017 began surveillance flights over the East China Sea to disrupt the transfers. The U.S. has conducted more than 300 surveillance flights since then, and allied nations have flown more than 200 surveillance flights since April 30, a U.S. defense official told NBC.

The U.S. and its allies increased its surveillance in September with the eight-nation coalition, including Australia, Britain, Canada, France, Japan, South Korea and New Zealand.

The coalition naval forces at sea have caused smugglers to stop ship-to-ship transfers in 30 instances.

“We've increased pressure and have been collecting information on these illicit transfers and then feeding them back to our interagency partners for financial, law enforcement and diplomatic action,” the official told NBC.

The expanded surveillance was in response to a sharp increase in ship-to-ship oil transfers this year, including 89 deliveries to North Korea’s ports between January and May, then-U.S. Ambassador to the U.N. Nikki Haley Nimrata (Nikki) Haley'The soul' versus 'law and order' Author Ryan Girdusky: RNC worked best when highlighting 'regular people' as opposed to 'standard Republicans' GOP lobbyists pleasantly surprised by Republican convention MORE said in July.

“It's a sustained effort but I would tell you the North Koreans are learning, evolving, getting better so the ship-to-ship transfers are taking place farther away from the Peninsula,” Randy Schriver, assistant secretary of Defense for Asian and Pacific security affairs, said at a discussion this month at the University of Chicago Institute of Politics.

The Treasury Department, meanwhile, continues to move forward on sanctions against North Korea. In recent months, the Treasury has blacklisted individuals, ships and companies accused of violating U.S. sanctions against North Korea. It has also given warnings to businesses and insurance companies to stay away from vessels or organizations suspected of taking part in smuggling operations.

Updated at 1:59 p.m.