The Trump administration on Friday levied sanctions against two foreign-owned companies and one tanker involved in the regular shipment of Venezuelan oil to Cuba, the latest in escalating measures to bring pressure on Venezuelan President Nicolás Maduro.

The sanctions block all U.S.-based assets of the two companies — one based in Liberia and the other in Greece — and prohibit any dollar transactions by the firms, along with the Liberian-flagged tanker. An additional 34 ships owned by Venezuela’s state oil company, ­PDVSA, were also blocked.

A senior administration official emphasized that the measures were only “the first step” to prevent Venezuela from shipping oil, its primary asset, overseas.

“We are continuing to tighten the noose of the financial strangulation of Maduro and his cronies,” said the official, who spoke to reporters on the condition of anonymity under ground rules set by the administration.

The sanctions are aimed as much at Cuba as Venezuela. For the past 20 years, the two countries have had an arrangement whereby Venezuela sends free and subsidized oil to financially strapped Cuba in exchange for the services of Cuban doctors, teachers, and military and security agents.

The administration has charged that thousands of Cubans are embedded throughout the Venezuelan military and intelligence agencies, where they control and spy on Venezuelan officers.

U.S. officials have expressed frustration at the failure of more than a trickle of Venezuelan security forces to defect from the Maduro government to support Juan Guaidó, the president of the opposition-controlled National Assembly, whom the United States and dozens of other countries in late January recognized as interim president.

Although Venezuela once supplied Cuba with as much as 100,000 barrels of oil per day, that has dropped precipitously, to 50,000 barrels or less. The opposition, still powerless to enforce its will, voted last month to stop the Cuba deal completely, and Guaidó, the administration official said, had requested the sanctions.

U.S. sanctions imposed earlier this year prevent PDVSA from collecting money for any oil shipped to the United States, which once imported more than a third of the company’s exports.

Overall Venezuelan oil exports have fallen to around 500,000 barrels a day. Much of it goes to Russia and China, although India has been the largest importer in recent months.

The immediate goal of the sanctions, the administration official said, is to put pressure on the Cuba-Venezuela partnership. “There is going to be a recalibration of that relationship,” the official said.

A broader objective is to “make things more radioactive” in the shipping industry, said Russ Dallen, a Florida-based managing partner at the brokerage Caracas Capital Markets.

“Most ships, ones not owned by Cuba or Venezuela, are like Uber drivers,” Dallen said. “The computer says, ‘Here’s the closest ship,’ and you book it.” In the future, “anybody who goes to Venezuela to pick up or drop off” oil, or goes to Cuba, “is going to be wary that they’re going to be sanctioned.”

The newly sanctioned companies are Ballito Bay Shipping, based in Monrovia, Liberia, and ProPer In Management, based in Piraeus, the Athens port. Ballito owns the tanker, the Despina Andrianna, and ProPer is its operator, according to a Treasury Department statement.

In recent months, the ship has made regular deliveries from Venezuela’s Jose offshore loading dock to Cuba. According to shipping trackers, it currently is located south of Cuba, off the northern shore of Jamaica, en route to Jose.

Tracking information lists no previous port of call for the ship. Dallen said ships carrying Venezuelan oil have begun turning off their transponders to make locating them more difficult — a tactic Iran also has used to avoid sanctions.

The administration has said its current policy to push Maduro from office is centered on diplomatic and economic measures.

“There is also a military option, which President Trump has said is on the table and remains on the table,” the administration official said, calling it “obviously, a very serious option . . . that no one would likely to see, but clearly one that is seriously considered as events continue to unfold.”

Maduro has arrested senior opposition officials and threatened Guaidó, whom the Maduro-controlled Supreme Court has banned from leaving the country. Guaidó has called for supporters to take to the streets Saturday in a major anti-government demonstration.

Similar actions in the past have led to violence as masked paramilitary forces, called “colectivos,” have attacked demonstrators.

“We’re watching very closely tomorrow,” the official said, calling on the Venezuelan military to “uphold their constitutional duty” to protect Venezuelans against what he called colectivo “terrorists.”