Key oil freight rates from the Middle East to Asia rocketed as much as 28% on Friday in a global oil shipping market spooked by United States sanctions on units of Chinese giant COSCO for alleged involvement in ferrying crude out of Iran.

In what the State Department called “one of the largest sanctions actions the U.S. has taken” since curbs were re-imposed on Iran in November last year, two units of COSCO were named alongside other companies in claims of involvement in sanctions-busting shipments of Iranian oil.

The surprise move, affecting one of the world’s largest energy shippers, operating more than 50 supertankers, comes as President Donald Trump seeks to exert maximum pressure on Iran to drop nuclear programs.

As some Asian oil buyers rushed to the shipping market to secure vessels, rates for chartering supertankers, or Very Large Crude Carriers (VLCCs), to load crude oil from the Middle East to north Asia in October surged nearly 19% overnight to about 75-76 points on Worldscale, an industry tool used to calculate freight charges, shipping and industry sources said.

That means an increase of about $600,000 for each ship, a Singapore-based crude oil trader said.

The rates for loading Middle East crude to west coast India in the second week of October jumped 28% to 80-92.5 points after Reliance Industries Ltd booked two supertankers overnight, industry sources said.

But there was also uncertainty over how widely the sanctions on the COSCO units - COSCO Shipping Tanker (Dalian) Co, Ltd and its subsidiary COSCO Shipping Tanker (Dalian) Seaman & Ship Management Co, Ltd - will be implemented. Industry sources said some oil buyers were holding off hiring COSCO tankers while they check with legal teams to better understand the impact of the sanctions.

“The market is fearful of sanctions so refiners are taking some preventive measures. We’ll have to see how widely implemented the sanctions will be,” said KY Lin, spokesman for Taiwanese refiner Formosa Petrochemical, a major crude oil buyer in Asia.

Friday’s jolt left shipping rates springing back to levels not seen since mid-September drone and missile strikes on key Saudi Arabian oil production facilities roiled global markets. The COSCO vessels are equal to about 7.5% of the world’s fleet of supertankers, according to Refinitiv data.

“Charterers are in trouble,” a North Asian shipbroker said, declining to be named citing company policy. “It was terrible news for every one of us with the Saudi drone attack, and now the market has to deal with U.S. sanctions on COSCO.”

“Good news for owners, good time for them to earn money,” the broker said.

While diplomatic tensions between the United States and Iran remain high, a British-flagged tanker that had been detained by Iran in the Strait of Hormuz on Friday left Bandar Abbas port heading for international waters.