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 US and backed by the United Nations. President Donald Trump's administration has vowed to maintain its "maximum pressure" campaign on North Korea even as America's ally, South Korean President Moon Jae-in, pursues a fragile detente sparked by the Olympics.