US unveils largest sanctions yet on ships, companies trading with North Korea U.S. officials did not rule out a military blockade of North Korean vessels.

 -- The Trump administration has slapped 27 trading and shipping companies, 28 vessels, and one individual with new sanctions for evading U.S. and United Nations embargoes on trading oil, coal, and other fuel with North Korea, and warned it might even impose a military blockade to stop North Korea's nuclear and ballistic missile programs.

President Donald Trump was supposed to reveal the sanctions himself in a speech Friday morning before the Conservative Political Action Conference, or CPAC, a major conservative summit, with the administration touting it as a major announcement. Instead, just as he wrapped up a more than one-hour-long speech, he mentioned it as an aside.

"I do want to say it, because people have asked -- North Korea -- we imposed today the heaviest sanctions ever imposed on a country before," he said. "Frankly, hopefully, something positive can happen. We will see."

Trump's daughter and senior adviser Ivanka Trump, who is in Seoul, South Korea, to attend the closing ceremonies of the Winter Olympics as the U.S. representative, briefed South Korean President Moon Jae-in on the sanctions, according to a senior administration official. They also discussed the continued effort on the joint maximum pressure campaign against North Korea, the official said.

Deemed the largest package of sanctions to date by the Treasury Department, it's the latest move by the U.S. in its global pressure campaign to force North Korea to give up its nuclear and ballistic missile programs, as the country marches on to developing a nuclear-tipped intercontinental ballistic missile.

The Treasury Department, along with the U.S. Coast Guard and the State Department, also issued a worldwide advisory, warning countries of "significant sanctions risks" if they keep doing business with North Korea. It's a threat the Trump administration has made in statements before, but such a formal warning paired with today's actions was meant to send a message that the U.S. wants to tighten the noose.

To that end, the Trump administration is not ruling out the idea of a military blockade to confront North Korean vessels in the event the sanctions don't affect their activity, according to Treasury Secretary Steve Mnuchin.

The U.S. mission to the U.N. will also seek U.N. sanctions on this same list of ships and companies, according to senior administration officials who briefed reporters. Doing so would really give the new move punch, but it's unclear if North Korea's backers China and Russia would sign on to those.

So far, those two have worked with the U.S. and the rest of the U.N. Security Council to pass three big resolutions last year, banning over 90% of North Korea's exports and capping its refined fuel imports. That means gasoline, diesel, and other fuels are down 89% now, according to a senior administration official, in the hopes of slowing their rocket development.

But in the face of those restrictions, North Korea has stepped up ship-to-ship transfers and other evasive tactics, according to a senior administration official.

"We know that North Korea is feeling the pressure," they said, but, "We also want to make sure that we are sensitive to the likelihood that North Korea is going to move toward more evasive actions... [which] we know North Korea is quite practiced at."

Transfers at sea may be far from the eyes of port authorities, but they do come under the watch of satellite images. The Treasury also released two images today, showing what they say are a North Korean ship falsifying its vessel information and the same ship conducting a transfer, likely of oil, with a Panama-flagged ship in December.

The new sanctions are meant to take on that activity and "make sure that the significant reductions in fuel going into North Korea that are mandated by the UN sanctions are unable to be circumvented," as one senior administration official said. The officials declined to say how much of an impact these specific sanctions will have, but Treasury said each of the nine vessels from countries other than North Korea could carry over $5.5 million worth of coal at a time.

But a top Democrat urged the administration to do more, saying, "the so-called 'largest package' of sanctions... yet again fails to apply the kind of targeted economic pressure necessary to bring Kim Jong Un to the negotiating table."

"China, the biggest enabler of North Korea’s destabilizing activities, only gets a slap on the wrist, escaping any punishment in this package. It will continue to pump crude oil into North Korea with little fear of an American-led oil embargo," said Sen. Ed Markey, D-Massachusetts, ranking member of the Senate Foreign Relations Committee's East Asia Subcommittee.

Despite Trump putting Russia on notice last month, there are no Russian entities on Friday's list either.

"Russia is not helping us at all with North Korea," Trump told Reuters in January, with Secretary of State Rex Tillerson adding, "They're not fully implementing all of the sanctions, and there’s some evidence that they may be frustrating some of the sanctions," especially those on fuel.

Instead, the ships and companies blacklisted today are from North Korea, China, Hong Kong, Singapore, Taiwan, Marshall Islands, Tanzania, Panama, and Comoros -- and the one individual is a Taiwanese businessman.

Trump was supposed to announce the sanctions himself in a speech before the Conservative Political Action Conference, or CPAC, a major conservative summit, Friday morning, with the administration touting it as a major announcement. Instead, just before wrapping up, he mentioned it as an aside.

"I do want to say it, because people have asked -- North Korea -- we imposed today the heaviest sanctions ever imposed on a country before," he said. "Frankly, hopefully, something positive can happen. We will see."

For his part, Vice President Mike Pence hailed the announcement as Trump 'delivering' after Pence promised "the toughest and most aggressive round of economic sanctions on North Korea ever" during a press conference with Japanese Prime Minister Shinzo Abe two weeks ago. The sanctions were not ready in time to be unveiled on Pence's trip to Japan and South Korea ahead of the Winter Olympics, according to Mnuchin.

ABC News's Alex Mallin contributed to this report.