MEXICO CITY/LONDON (Reuters) – Oil tankers that were sailing toward Venezuela have turned around and others have left the country’s waters as the United States considers blacklisting dozens of ships for transporting Venezuelan oil, according to shipping data and industry sources.

The threat of tighter sanctions is already disrupting the global shipping market. Chinese oil firms are considering whether to decline to charter any tanker that has visited Venezuela in the past year, no matter where the ship is now or for what voyage, four shipping sources told Reuters on Tuesday.

Washington is seeking to oust the socialist government of President Nicolas Maduro by choking the oil exports that provide its main source of income. The measures have contributed to a fall in Venezuelan oil exports to a 17-year low and deepened the country’s economic crisis, but Maduro has held on – to the frustration of U.S. President Donald Trump’s administration.

The United States may add dozens more tankers to an existing blacklist, U.S. sources told Reuters last week. That would make it more difficult for state firm PDVSA to deliver oil to refineries abroad. Exports dropped to about 452,000 barrels per day in May, the lowest since a national strike paralyzed the economy and hit exports between 2002 and 2003.

Venezuela’s Foreign Minister Jorge Arreaza said on Twitter on Tuesday that Washington was attacking Venezuela’s economy by blocking foreign revenue that could be used to import humanitarian goods, including food and medicine. The U.S. State Department did not immediately comment.

Malta-flagged tanker Seadancer, operated by Greek firm Thenamaris Ships Management and chartered by Thai refiner Tipco Asphalt, returned to Gibraltar after waiting in the Atlantic Ocean for a week, according to Refinitiv Eikon tracking data.

Tipco Asphalt told Reuters on Tuesday the company had dropped plans to use the vessel, which had previously been on course for waters off the coast of Venezuela at Amuay, according to the Eikon data and PDVSA’s export schedules.

The Seadancer was scheduled to load 1 million barrels of Venezuela’s Boscan crude for shipping to Malaysia’s Kemaman refinery, operated by Tipco, according to the schedules.

Another vessel operated by Thenamaris that had loaded crude in Venezuela in February, the Seahero, was blacklisted by the U.S. Treasury Department last week.

A second Malta-flagged tanker expected in Venezuela, the Novo, made a U-turn this week in the Caribbean. The vessel was scheduled to transport 1 million barrels of Hamaca crude in June to Singapore, Eikon and PDVSA data showed.

A loading window assigned for the Novo was canceled on June 6 after the chartering contract was suspended, according to the PDVSA schedules reviewed by Reuters. The tanker is operated by Dynacom Tankers Management Ltd, which also manages the Chios I, blacklisted last week by the U.S. Treasury.

PDVSA, Thenamaris Ships Management and Dynacom Tankers did not immediately reply to requests for comment.

As word spread on the possibility of more Venezuela-related shipping sanctions to come, at least three more very large crude carriers (VLCCs) – Boston, Commodore and Respect – exited Venezuelan waters over the weekend to anchor in the Eastern Caribbean, the Eikon data showed.

Oil companies and merchants worldwide – not just in China – are becoming more wary of vessels that have recently transported Venezuelan oil, the sources said.

“Anything on the potential sanctions list will just become toxic,” a source at a top oil trading company said. “No one will touch it until it’s clear what the rules will be.”

Broker Clarksons Platou Securities estimated that 77 tankers had called at Venezuela’s main oil ports since December, more than 2% of the global fleet, and so were potentially at risk of sanctions.

Sanctions typically have a knock-on effect on the rest of the oil tanker market as energy companies and merchants scramble to swap blacklisted vessels for others.