The US has boosted pressure against Iran's ability to export oil, identifying Tehran's main tanker firm and exposing dozens of its vessels as government-controlled entities to now be subjected to Western sanctions.
The US treasury department identified the National Iranian Tanker Company (NITC), 58 of its vessels and 27 of its affiliates as extensions of the state, which would undermine Iran's attempts to use renamed, disguised vessels to evade sanctions imposed over its nuclear programme.
The fresh targets announced on Thursday, which also included naming what Washington said were four front companies for Iran's state oil enterprise, would help countries and foreign companies comply with Western penalties against Iran.
An Obama administration official said the measures would have some impact on Iran's ability to sell oil. "It will make it that much more difficult for Iran to deceive potential purchasers about the origin of the oil", the official told reporters.
US companies and Americans are already prohibited from doing business with entities controlled by Iran's government.
The National Iranian Tanker Company changed the names and flags of many of its oil tankers ahead of a European Union ban on Iranian oil imports. That included swapping Maltese and Cypriot flags for Tuvalu and Tanzanian ones.
The main part of NITC's oil fleet can carry a maximum of around 62 million barrels of oil, data from its website show.
The fleet has become significantly more important this month because new EU sanctions have cut off access to the London-based ship insurance market, putting Iran off-limits to almost every major tanker firm.
The US has mounted an international campaign aimed at depriving Iran of oil revenue to pressure it to rein in its nuclear programme, which Tehran maintains is solely for peaceful purposes.
"We will continue to ratchet up the pressure so long as Iran refuses to address the international community's well-founded concerns about its nuclear program," David Cohen, US treasury undersecretary, said in a statement.
The US sanctions have limited Iran's major trading partners from buying Iranian crude. The European Union banned Iranian oil imports as well as providing insurance for vessels carrying Iranian oil from July 1.
Malaysian-based Noor Energy, Petro Suisse, Dubai-based Petro Energy, and Hong Kong Intertrade were identified as being controlled or acting on behalf of the Iranian government.
The US treasury department said they were acting as front companies for the National Iranian Oil Company (NIOC) and other blacklisted Iranian entities.
US lawmakers said the treasury's action was a move in the right direction but said much more had to be done.
"We must continue to increase pressure on the Iranian regime until it verifiably abandons its nuclear weapons program," said Howard Berman, the top Democrat on the House Foreign Affairs Committee, who has been asking Tuvalu and Tanzania to stop reflagging Iranian oil tankers.
A senior US senate aide said the administration was finally playing the game correctly by exposing Iranian fronts for sanctions evasion and laying the groundwork for new sanctions legislation that will make any business dealing with such entities illegal.
The US department also imposed economic sanctions on 11 entities including Iran's ministry of defence logistics export, which represents the country at arms trade fairs.
In addition, four individuals including an Austrian who allegedly provided support to Iran's missile programme and military were blacklisted.
The penalties will bar US companies and Americans from doing business with them and freeze any assets they may have in the United States.