The Trump administration has escalated pressure on North Korea by slapping sanctions on scores of companies and ships accused of illicit trading with the pariah nation.
Treasury Secretary Steven Mnuchin said the US has now blacklisted virtually all ships being used by the North.
The administration billed it as the largest instalment 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 at the Conservative Political Action Conference that it was "the heaviest sanctions ever imposed on a country before".
While that claim was questionable - previous US measures have targeted bigger players in the North Korean economy, including Chinese and Russian banks and business networks - it significantly tightens the noose on North Korean trading.
Mr Mnuchin told reporters that the US 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".
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 that pose an emerging threat to the US mainland.
Washington is particularly concerned about exports of North Korean coal that are prohibited by the UN sanctions and ship-to-ship transfers of imported oil and petroleum products.
The Treasury Department said it was barring US 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.
Mr Tsang was said to have co-ordinated North Korean coal exports with a Russia-based North Korean broker, and attempted a $1m oil deal with a Russian company sanctioned for dealing with the North.
Mr 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 US 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 US and UN sanctions as a national security imperative. Those who trade with North Korea do so at their own peril," Mr Mnuchin said.
In his speech, Mr Trump said "hopefully something positive can happen" from the sanctions pressure.
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 US ally, animosity between Washington and Pyongyang still runs high.
Ivanka Trump, the president's daughter and adviser, arrived in South Korea today 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 denuclearised".
Mr Mnuchin said Mrs Trump had spoken to Mr Moon about the new sanctions before the announcement.
The US government also issued 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 US-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 on Friday.