President Trump announced new financial measures against North Korea on Friday aimed at ensuring Pyongyang cannot continue to fuel its nuclear ambitions by sidestepping existing sanctions.

“Today we put the strongest sanctions on Korea that we have ever put on a country,” Trump said at a news conference with the Australian prime minister. “We must continue to stand together to prevent the brutal dictatorship from threatening the world with nuclear devastation.”

The measures target 56 vessels, shipping companies and other entities that U.S. officials say are used by North Korea to conduct trade prohibited under previous sanctions.

They come as the Trump administration seeks new ways to intensify pressure on North Korean leader Kim Jong Un, whose increasingly advanced missile and nuclear weapon programs have made the isolated nation the most pressing foreign threat facing the United States.

The penalties, the latest of multiple rounds of Trump administration sanctions, signal U.S. officials’ concern that an under-the-radar fossil fuel trade undermines their ability to use economic pain as a means for making the Kim regime rethink its nuclear program.

Trump first referenced the new sanctions in the final moments of a more-than-hour-long speech at the Conservative Political Action Conference outside Washington.

“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, briefing reporters ahead of Trump’s address, said the steps North Korea has taken to conceal its illicit shipping activities include conducting prohibited ship-to-ship transfers, falsifying ship names and disabling vessels’ automatic identification systems “to try to intentionally 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,” said one official, who, like others, spoke on the condition of anonymity because they were not authorized to speak publicly.

Experts said new sanctions would not have the same economic bite as previous measures targeting North Korean exports but may ensure better enforcement by disrupting the increasingly sophisticated steps Pyongyang takes to evade them.

Nine of the sanctioned ships were capable of carrying large shipments of coal, a vital North Korean export that was tagged by earlier measures.

“These will incrementally add to the pressure but not make a substantive difference,” said Patrick M. 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.”

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

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

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

Stephen Osborne, who has written about maritime sanctions as a research associate at Project Alpha, a nonproliferation group based at King’s College London, said the new measures would make clear to other countries which companies to avoid.

“The success of the illicit trade relies on secrecy, and the most effective approach against that is exposure,” Osborne said. “To give a clear outline of the vessels, individuals and companies that have been linked to North Korea is a good first step.”

Treasury Secretary Steven Mnuchin, in a briefing, said the actions would put companies and countries “on notice that this administration views compliance with U.S. and U.N. sanctions as a national security imperative. Those who trade with North Korea do so at their own peril.”

Speaking beside Australian Prime Minister Malcolm Turnbull, Trump suggested that the United States might resort to military action if sanctions don’t work.

“We’ll have to see,” Trump said. “I don’t think I’m going to exactly play that card. But we’ll have to see. If the sanctions don’t work, we’ll have to go Phase Two. And Phase Two may be a very rough thing — it may be very, very unfortunate for the world. . . . If we can make a deal, it will be a great thing. And if we can’t, something will have to happen.”

The announcement takes place as Ivanka Trump and a presidential delegation visit South Korea for the Closing Ceremonies of the 2018 Winter Olympic Games.

Trump attended a dinner earlier Friday with South Korean President Moon Jae-in. Mnuchin said Trump was briefed on the U.S. move, and she discussed the sanctions with Moon before they were publicly announced.

Moon said North Korea’s participation in the 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.

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

Speaking before the announcement, Nikki Haley, the U.S. ambassador to the United Nations, claimed success for the United States 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 Washington can apply on its own, mostly because they involve North Korea’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 in an address to the University of Chicago’s Institute of Politics. “The regime has less and less money to spend on its ballistic missile tests and less capacity to threaten other countries with those tests.”

David Nakamura and John Wagner in Washington, Anne Gearan in Chicago and Anna Fifield in Seoul contributed to this report.