DANDONG, China — As the end of the fashion season approached, and the suits and dresses arrived in her company’s warehouses here in the Chinese border town of Dandong, the accountant crammed about $100,000 into a backpack, then boarded a rickety train with several co-workers.

She asked to be identified only by her surname, Lang, given the sensitivity of their destination: North Korea.

After a six-hour journey, she recalled, they arrived at a factory where hundreds of women using high-end European machines sewed clothes with “Made in China” labels. Her boss handed the money to the North Korean manager, all of it in American bills as required.

Despite seven rounds of United Nations sanctions over the past 11 years, including a ban on “bulk cash” transfers, large avenues of trade remain open to North Korea, allowing it to earn foreign currency to sustain its economy and finance its program to build a nuclear weapon that can strike the United States.