Recent U.N. and U.S. watch lists and Japanese surveillance reports have identified ships and companies suspected of helping North Korea evade sanctions for its nuclear weapons program.

The reports also reveal connections to Hong Kong through front companies based in the southern Chinese financial center.

That has underscored rising concern over Hong Kong's role as a nexus for North Korea's underground business network, and prompted the U.S. government to urge the territory's authorities to crack down.

The United Nations panel that monitors North Korean sanctions said in a recent report that the corporate registration agents that set up these front companies "present a key vulnerability in the implementation of financial sanctions."

It said investigations found that North Korean-linked companies used evasion tactics including Hong Kong front companies.