“We’ve been messaging very heavily to the shipping companies, you don’t want to do this, it’s not worth it,” said one. “It’s incredibly dangerous and irresponsible behavior.”

The International Maritime Organization, the UN agency that regulates shipping, requires that AIS be used but crews can turn off the system to hide a ship’s movements.

“We are literally going ship by ship at this point because each ship is incredibly important” an official said.

China is the largest remaining buyer of Iranian oil after US President Donald Trump reimpose sanctions on Tehran’s main export. Trump tightened US sanctions in May to drive Iran’s oil sales to zero.

The sanctions are aimed at quashing Iran’s nuclear ambitions, ballistic missile program and influence in Syria, Iraq and other countries. Its oil exports have fallen to less than 300,000 barrels per day from about 2.5 million bpd.

On September 25, the US imposed sanctions on five Chinese individuals and two Chinese COSCO Shipping Corp subsidiaries, saying they had shipped Iranian crude oil violation of the sanctions.

Analysts believe Iran could be trying to weather the sanctions in the hope that the 2020 US presidential elections will bring in a new administration and an easing of Trump’s “maximum pressure” policy.

“If they calculate that there is going to be (a new president) and they are incorrect, that’s bad,” one of the officials told Reuters. “Because I don’t think anybody thinks that they could make it another four years after next November under this kind of sanctions regime.”