Three tankers carrying Russian diesel are crossing the Atlantic toward U.S. ports, with a fourth set to depart soon, as President Donald Trump's fuel deal with Moscow draws opposition in Congress and threats to investigate companies involved in the shipments.

This came as a former Treasury Department sanction official confirmed to Fox News Digital that Trump has the legal authority to allow the imports despite a 2022 law banning Russian energy products, arguing that Washington's authorization does not eliminate the underlying prohibition.

"The concern that the president has violated a law from 2022 that prohibits the importation is actually not founded in law, so the president certainly has the authority in this case to provide the authorization," Jeremy Paner, a partner at Hughes Hubbard & Reed, told Fox News Digital.

Russian Deputy Prime Minister Alexander Novak also confirmed the tanker movements Tuesday on the sidelines of the Gas Exporting Countries Forum (GEC) summit, saying a fourth vessel was expected to depart Russia Oct. 28. "There are three ships at the moment. The fourth has not yet departed," Novak said, according to TASS.

The shipments, totaling around 140,000 metric tons, are the first authorized cargoes under an Oct. 9 agreement between Trump and Russian President Vladimir Putin temporarily permitting imports of Russian diesel.

The administration followed the process to ease diesel prices amid supply disruptions linked to the war with Iran and Ukrainian strikes on Russian refineries.

The deal triggered opposition on Capitol Hill, where lawmakers argue the administration lacks the authority to bypass the Ending Importation of Russian Oil Act of 2022 without congressional approval.

Sen. Richard Blumenthal, D-Conn., also escalated opposition on Tuesday, demanding that two shipping companies recall tankers carrying Russian diesel bound for U.S. ports.

In letters to Eastern Mediterranean Maritime Limited and PetroVietnam, Blumenthal argued that the Treasury Department's General License No. 135 cannot override the 2022 import ban and warned that the deliveries would violate federal law.

He also demanded details about the companies involved in the shipments and any federal guidance they received by Oct. 31. Paner however, drew a distinction between removing sanctions and authorizing specific transactions.

"Trump's action did not lift, or remove, or waive the underlying prohibition on the importation of Russian diesel into the United States," Paner said.

"What it did was authorize it, and as a matter of law, that's different. A license does not get rid of a prohibition. It's simply allowed," the expert added.

The incoming tankers have drawn scrutiny because at least two are subject to Western sanctions, according to reports.

The first tanker, the Malta-flagged Agioi Fanendes, is carrying about 40,000 metric tons of diesel and signaling New York as its destination, with an estimated arrival date of Nov. 7.

A second vessel, the 20-year-old Talisman, is expected to reach the East Coast on Nov. 9 and is sanctioned by the U.S., United Kingdom and the EU.

The third tanker, the Sierra Leone-flagged, Hong Kong-owned Leruo, is signaling an East Coast arrival around Nov. 11.

The Leruo was sanctioned in 2025 by the European Union, United Kingdom and Canada over alleged deceptive shipping practices involving Russian oil and petroleum products and conducting ship-to-ship transfers at sea.

Paner said the U.S. authorization would allow the shipments to proceed under American sanctions rules, even if a vessel is separately designated under U.S. sanctions.

"From a U.S. legal perspective, U.S. sanctions do not get in the way of importing Russian diesel from these three ships, even though one of the ships is sanctioned by the United States and even though the importation of Russian diesel is prohibited by U.S. law," Paner explained.

"Because of the breadth of the authorization that OFAC issued, it would cover anything that would touch upon Russian sanctions," Paner said, referring to the Treasury Department's Office of Foreign Assets Control.

"From a U.S. perspective, it's not relevant that one of these three vessels is sanctioned by the U.S.," he added.

But Paner warned that American authorization would not automatically protect foreign companies operating under other countries' sanctions regimes.

"Where it gets complicated is the EU, U.K. and Canadian sanctions on some of the vessels, and that is because there are so many different players in the maritime industry, including shippers, insurers, re-insurers and banks," Paner said.

"There could be European operators involved, and the U.S. authorization would not authorize those European, U.K. or Canadian companies to have any involvement in these shipments." The complications could explain why sanctioned vessels are being used for the deliveries, according to Paner.

"Sanctioned vessels would be used because of the reputational and legal risk exposure from moving Russian diesel," he said.

"Even if it's blessed by the U.S., there are other sanctions out there that these big global logistics companies and shipping companies have to keep in mind as well," Paner added.

He also cautioned that vessel ownership could complicate efforts to determine which sanctions apply.

"I don't know who owns them currently. I don't know if the same entity identified by the various sanctioning authorities owns them now." The shipments underscore a shift in Washington's approach to Russian energy restrictions imposed following Moscow's 2022 invasion of Ukraine, when the United States banned Russian petroleum imports and imposed price caps on Russian oil and refined products.

Under the deal, Washington authorized certain Russian diesel imports in exchange for Moscow releasing 300,000 metric tons to global markets, rising to 500,000 in November and potentially 1 million per month after.

Novak also criticized lawmakers opposing the arrangement, accusing them of acting against the interests of American consumers.

"Overall, it is certainly surprising to see the stance taken by Democrats, who speak out — or even take action — in a way that is not in the best interests of the residents of the United States of America," Novak said.

In an Oct. letter to commercial banks, fuel distributors and port authorities in New York, New Jersey and along the Texas Gulf Coast, lawmakers also warned that companies involved in the diesel shipments could face investigations.

They also demanded that companies preserve transaction records, port reservations and communications, giving recipients until Oct. 28 to confirm they had taken steps to retain the documents.

"I'm confident that Congress is going to investigate on the U.S. side, and then from investigating the U.S. involvement, they'll be able to understand the non-U.S. companies that are involved as well," Paner concluded.