(Reuters) — Seeking to deter further shipments of Iranian fuel to Venezuela, the Trump administration has quietly warned foreign governments, seaports, shipping companies and insurers that they could face stiff U.S. sanctions if they aid the tanker flotilla, the U.S. envoy on Venezuela told Reuters on Friday.

Elliott Abrams, Washington’s special representative on Venezuela, said the pressure campaign targeting heavily sanctioned U.S. foes Iran and Venezuela was being waged “to be sure everyone recognizes this would be a very dangerous transaction to assist.”

The Venezuelan navy on Thursday escorted a fourth tanker bringing Iranian fuel through its waters to the gasoline-starved country, defying U.S. threats of “measures” in response to the shipments. At least one other tanker was en route in the Atlantic.

It was a sign of deepening ties between Venezuela and Iran, both OPEC members with fraught relations with the United States.

The government of Venezuelan Socialist President Nicolas Maduro has flaunted the tankers’ arrivals to show it remains unbowed by pressure. The United States, which seeks President Maduro’s ouster, has called it a “distraction.”

“We've alerted the shipping community around the world, ship owners, ship captains, ship insurers, and we've alerted ports along the way between Iran and Venezuela,” Mr. Abrams said in an interview.

He said diplomatic warnings, known as demarches, have been sent privately to governments “around the world.”

A person familiar with the matter said among them was Gibraltar, situated on the tankers' route. A U.S. official said various countries had been asked to deny then port services.

It remained unclear what impact this effort might have.

Two other tankers, the Liberia-flag Bella and Bering, passed through the Suez Canal in early May, according to Refinitiv Eikon data. Sources familiar with the matter said United States was looking for ways to prevent them from reaching Venezuela.

Mr. Abrams declined comment but, citing the sanctions risk, said: “I do not think that you will find ship owners and insurers and captains and crews willing to engage in these transactions in the future."

He predicted the fuel would only last a few weeks and mostly be siphoned off by President Maduro loyalists. Shortages have grown acute due to U.S. sanctions against Venezuela under President Maduro, who has overseen an economic collapse.

The United States this month issued a global maritime advisory, giving guidance to the shipping industry on how to avoid sanctions related to Iran, North Korea and Syria.

Washington has appeared willing to rely on economic measures instead of using its beefed-up naval presence in the Caribbean to block the tankers. Iran and Venezuela have warned against U.S. military force.