20 Million barrels of Iranian crude sits at China's Dalian Port.

Some 20 million barrels of Iranian oil sitting on China's shores in the northeast port of Dalian for the past six months now appears stranded as the United States hardens its stance on importing crude from Tehran. Iran sent the oil to China, its biggest customer, ahead of the reintroduction of U.S. sanctions last November, as it looked for alternative storage for a backlog of crude at home.

The oil is being held in so-called bonded storage tanks at the port, which means it has yet to clear Chinese customs. Despite a six-month waiver to the...