WASHINGTON - President Donald Trump announced the "heaviest" ever set of U.S. sanctions against North Korea on Friday as his administration redoubles its effort to starve Pyongyang of resources and force the isolated regime to abandon its nuclear ambitions.

"We imposed today the heaviest sanctions ever imposed on a country before," Trump said at the close of a wide-ranging address to a conservative political conference.

The new measures target 56 vessels, shipping companies and other entities that Trump administration officials believe are used by North Korea to conduct trade prohibited under previous sanctions.

Significant advances in its missile and nuclear programs have made North Korea the most pressing foreign threat facing the Trump administration.

The punitive measures, which follow multiple rounds of earlier sanctions, signal U.S. officials' determination to use financial pressure to prompt North Korean leader Kim Jong Un to relinquish nuclear weapons, and their concern that under-the-radar fossil fuel trade undermines their ability to do so.

Trump made the announcement in the final moments of a more than hour-long address at the Conservative Political Action Conference outside Washington. "I love you, I respect you, I appreciate everything you have done for the country," the president said, pausing as the crowd applauded and a few chanted "USA! USA!"

"I do want to say, because people have asked: North Korea," he said before briefly mentioning the sanctions. "Hopefully something positive can happen. We will see."

Senior U.S. officials described elaborate steps that North Korea has taken to conceal its illicit shipping activities, including conducting prohibited ship-to-ship transfers, falsifying ship names and disabling vessels' automatic identification systems "to try to intentional mask their movements."

"We are determined through these efforts to increase the pressure and show Kim Jong Un that there is no other path to take but denuclearization," one official, who like others spoke on the condition of anonymity, told reporters ahead of Trump's remarks.

It is unclear whether improved sanctions enforcement will significantly effect the Kim government, which has been defiant in the face of increasing economic pain.

"These will incrementally add to the pressure but not make a substantive difference," said Patrick Cronin, an Asia scholar at the Center for a New American Security. "The strategy is meant to be one of slowly increasing pressure, not a sudden destabilizing blow."

Also on Friday, the Treasury Department released photos of what it said were North Korean ships using fraudulent names and ID numbers.

The entities sanctioned include companies based in China, Singapore and Panama.

The administration issued a global shipping advisory designed to increase awareness of North Korea's attempts to hide its maritime activities and to put companies on notice that they could be subject to penalties if they trade with the regime.

"Through today's actions we are putting companies and countries across the world on notice that this administration views compliance with U.S. and U.N. sanctions as a national security imperative," Treasury Secretary Steven Mnuchin told reporters. "Those who trade with North Korea do so at that their own peril."

Mnuchin said the scope of this round of sanctions is considered "both the largest in number that we've ever done against them, as well as impactful."

The new measures come as Ivanka Trump and a presidential delegation visit South Korea to attend the 2018 Winter Olympic Games.

Trump attended a dinner earlier on Friday with South Korean President Moon Jae-in. Mnuchin said Ivanka Trump has been briefed on this, and she discussed the sanctions with Moon before they were publicly announced.

Moon said North Korea's participation in Olympics has "served as an opportunity for us to engage in active discussions between the two Koreas and this has led to lowering of tensions on the peninsula and an improvement in inter-Korean relations."

"I also believe that such developments are thanks to President Trump's strong support for inter-Korean dialogue, and I would like to express my deep appreciation on this point as well," Moon said.

Ivanka Trump, a senior adviser to the president who doesn't plan to make North Korea the main focus of her visit, said the Trump administration is committed to "our maximum pressure campaign to ensure that the Korean Peninsula is denuclearized."

Another key element of the Trump administration's strategy for confronting North Korea is building a more comprehensive regime of international sanctions.

Speaking hours before the new sanctions were announced, U.N. Ambassador Nikki Haley said the Trump administration had prevailed where its predecessors had failed.

"After 25 years of failed bipartisan attempts to bribe the North Koreans into ending their nuclear pursuit, we realized that we have to stop the revenue streams that support it," she said in an address to the University of Chicago's Institute of Politics.

She claimed U.S. success in steering three rounds of successively tighter international sanctions at the U.N. Security Council over the past year. Those penalties are more effective than any the United States can apply on its own, mostly because they involve Pyongyang's major trading partner, China.

"And even though North Korea has yet to end its nuclear and missile programs, we know the sanctions are having a real impact," Haley said. "The regime has less and less money to spend on its ballistic missile tests, and less capacity to threaten other countries with those tests."

She said the squeeze, more than any other factor, forced the Kim regime to "reach out to South Korea and do public relations damage control" at the Olympics.

"Their sources of revenue are drying up. Sending cheerleaders to PyeongChang was a sign of desperation, not national pride," she said.

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The Washington Post's David Nakamura in Washington, Anne Gearan in Chicago and Anna Fifield in Seoul contributed to this report.