WASHINGTON -- The U.S. announced Friday its largest sanctions to date against those suspected of supporting North Korea's nuclear and missile programs as part of its maximum-pressure campaign against the regime.

The measures target 28 vessels and 27 trading and maritime transport companies linked to North Korea, China, Singapore, Hong Kong, Taiwan, Panama, the Marshall Islands, Tanzania, and Comoros, along with one Taiwanese individual, according to a list released by the U.S. Treasury Department. The aim is to crack down on ship-to-ship oil transfers used by North Korea to evade United Nations sanctions, cutting off resources for nuclear development.

"We impose today the heaviest sanctions ever imposed on a country before," President Donald Trump said in a speech at the Conservative Political Action Conference.

Trump ratcheted up his rhetoric even further later that day, saying that if the sanctions do not work, the U.S. will have to "go phase two."

"Phase two may be a very rough thing, may be very, very unfortunate for the world, but hopefully the sanctions will work," he said at a joint news conference with Australian Prime Minister Malcolm Turnbull. Trump did not specify what that would mean.

The U.S. assets of targets are frozen and Americans are barred from dealing with them. Washington will also take further action to deter ships from concealing their identities, including their countries of registration, a senior official said.

The decision to expand the American sanctions during the Winter Olympics in South Korea appears meant to warn Seoul against growing too friendly with its northern neighbor. Pyongyang is certain to rail against the new measures.

North Korea is suspected of engaging in ship-to-ship oil transfers to evade international sanctions. (courtesy of the Japanese defense ministry)   © Kyodo

The Treasury Department imposed sanctions on North Korean vessels and shipping companies in late January. Vice President Mike Pence said during a visit to Japan this month ahead of the Olympics that even tougher steps would come soon.

Pence was originally set to meet with Kim Yo Jong, sister of North Korean leader Kim Jong Un, during the games at Pyongyang's request, but the North walked away at the last minute. Speaking Thursday at CPAC, Pence castigated Kim Yo Jong for "abetting North Korea's horrendous human rights abuses and crimes against humanity."