As Washington and Seoul try to maintain a unified front against North Korea, the case of two cargo ships shows how Kim Jong Un's regime keeps finding ways to evade increasingly tough international sanctions aimed at halting its nuclear weapons program.

Both vessels have gone through repeated changes of names and owners, part of an international shell game that has undercut the escalating sanctions led by the U.S. and backed by the United Nations. President Donald Trump's administration has vowed to maintain its "maximum pressure" campaign on North Korea even as South Korean President Moon Jae-in pursues a fragile detente sparked by the Olympics.

North Korea earned almost $200 million in the first nine months of last year from banned commodity exports, providing crucial foreign currency for the isolated regime, according to a confidential United Nations report, portions of which were seen by Bloomberg News. The panel of experts who wrote the study said Kim's regime had shipped coal to ports in Russia, China, South Korea, and Vietnam, mainly using false paperwork and front companies that concealed the origin of the coal.

The banned exports and imports are transported on vessels flagged by countries in Africa, the Caribbean and Hong Kong. Sanctioned ships take on new flags, new companies are created to mask ownership and the vessels get new identities. Then they keep plying the same waters. That was the case with the two ships, the Jin Teng and the Jin Tai 7.

The Jin Teng, sanctioned by the U.S. in March 2016, became the Shen Da 8 and then the Hang Yu 1 last November, according to Kharon, a Los Angeles-based firm that identifies sanctions risks for banks and companies. The Jin Tai 7, also sanctioned by the U.S. in March 2016, changed its name to Sheng Da 6 two months later and then to Bothwin 7 last November, Kharon said. That was before a new round of U.N. sanctions was agreed on in December. Both ships remain on the U.S. sanctions list.

Enforcing sanctions is the biggest challenge for the Trump administration. Last week, Treasury Secretary Steven Mnuchin announced a raft of new sanctions against 27 entities and 28 vessels, which he said would "significantly hinder North Korea's ability to conduct evasive maritime activities that facilitate illicit coal and fuel transports, and limit the regime's ability to ship goods through international waters."