WASHINGTON (AP) — The Trump administration escalated pressure on North Korea Friday by slapping sanctions on scores of companies and ships accused of illicit trading with the pariah nation. Treasury Secretary Steven Mnuchin said the U.S. has now blacklisted virtually all ships being used by the North.

The administration billed it as the largest installment of North Korean economic restrictions to date as it intensifies its campaign of “maximum pressure” to get the North to give up its nuclear weapons. President Donald Trump went further, declaring in a speech Friday at the Conservative Political Action Conference that it was “the heaviest sanctions ever imposed on a country before.”

While that claim was questionable — previous U.S. measures have targeted bigger players in the North Korean economy, including Chinese and Russian business networks — it significantly tightens the noose on North Korean trading. Mnuchin told reporters that the U.S. has now imposed more than 450 sanctions against the North, about half of them in the last year — including “virtually all their ships that they’re using at this moment in time.”

Trump, who has vowed to use force if necessary to prevent North Korea acquiring a nuclear-tipped missile that could strike the U.S. mainland, warned that if sanctions don’t work, the U.S. would move to “phase two” in its pressure campaign against Pyongyang. He told a White House news conference that could be “very rough” and “very unfortunate for the world.” He did not elaborate.

“It really is a rogue nation,” Trump said in reference to North Korea. “If we can make a deal it will be a great thing. If we can’t, something will have to happen.”

The United Nations Security Council has imposed three sets of sanctions on North Korea in the past year. The restrictions are intended to deprive it of revenue and resources for its nuclear and ballistic missile development. Washington is particularly concerned about exports of North Korean coal that are prohibited by the U.N. sanctions and ship-to-ship transfers of imported oil and petroleum products.

The Treasury Department said it was barring U.S. business transactions with nine international shipping companies from China, Hong Kong, Singapore and Panama, and nine of their vessels. It also blacklisted 16 North shipping companies and 19 of their North Korean-flagged vessels.

Additionally, the department designated a Taiwanese citizen, Tsang Yung Yuan, and two companies he owns or controls. Tsang was said to have coordinated North Korean coal exports with a Russia-based North Korean broker, and attempted $1 million oil deal with a Russian company sanctioned for dealing with the North.

Mnuchin said the actions will significantly hinder North Korea’s ability to conduct evasive maritime activities that facilitate illicit coal and fuel transports, and “erode its abilities to ship goods through international waters.” He vowed the U.S. would “do everything” to stop the ship-to-ship transfers.

“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. Those who trade with North Korea do so at their own peril,” Mnuchin said.

The announcement comes as South Korea hosts the Winter Olympics, an occasion the two Koreas have used as an opportunity to ease tensions and restart talks. Although South Korea is a close U.S. ally, animosity between Washington and Pyongyang still runs high and there’s no sign of the adversaries starting negotiations on the nuclear issue.

Ivanka Trump, the president’s daughter and adviser, arrived in South Korea on Friday to attend the closing ceremony this weekend. At a dinner with South Korean President Moon Jae-in, she reaffirmed “our commitment to our maximum pressure campaign to ensure that the Korean Peninsula is denuclearized.” Mnuchin said Ivanka Trump had spoken to Moon about the new sanctions before the announcement.

The U.S. government also issued Friday a global shipping advisory highlighting the sanctions risk to those who enable shipments of goods to and from North Korea. It alerted industries to North Korea’s “deceptive shipping practices,” which includes falsifying the identity of vessels and disabling transponders that track ships’ movements.

The Treasury Department published photos of a U.S.-designated North Korean vessel, Kum Un San 3, which it said used false identifying information and conducted an illicit ship-to-ship transfer, possibly of oil, with a Panama-flagged vessel that was among the ships sanctioned Friday.

Treasury Secretary Steve Mnuchin speaks during a press briefing at the White House in Washington, Friday, Feb. 23, 2018. The Trump administration announced new sanctions on more than 50 vessels, shipping companies and trade businesses in its latest bid to pressure North Korea over its nuclear program. (AP Photo/Pablo Martinez Monsivais)