The company denied it had ever sold ships to the Islamic state.
"We have never sold ships to Iran," it said in a statement, adding the Israeli government would support its claims.
The United States says Ofer Brothers sold a tanker to the Islamic Republic of Iran Shipping Lines (IRISL).
The US also imposed new sanctions on Venezuela's state oil company PDVSA and five other oil and shipping firms on Tuesday for engaging in trade with Iran in violation of a US ban.
Got help from Israeli firm? Ahmadinejad in Natanz reactor (Photo: AP)
The other companies are PCCI, the Royal Oyster Group and Speedy Ship of the United Arab Emirates, Tanker Pacific of Singapore and Associated Shipbroking of Monaco.
In the case of Ofer Brothers, the group was accused of providing a tanker valued at $8.65 million to the IRISL along with Tanker Pacific.
"We believe that Tanker Pacific and Ofer Brothers Group failed to exercise due diligence and did not heed publicly available and easily obtainable information that would have indicated that they were dealing with IRISL," the US State Department said in a statement.
"The Secretary (Hillary Clinton) will hold companies accountable, as required by the Iran Sanctions Act, when they know or 'should have known' they were providing sanctionable goods or services to Iran," it added.
With the imposition of sanctions, Tanker Pacific and Ofer Brothers Group are barred from securing financing from the Export-Import Bank of the United States, from obtaining loans over $10 million from US financial institutions and from receiving US export licenses.
The Ofers are Israel's richest family. They control Israel Corp, one of Israel's largest investment companies.
Israel Corp holds stakes in Israel Chemicals (ICL), the world's sixth-largest maker of potash, shipping firm Zim, chipmaker TowerJazz and Oil Refineries. It is also a main financial backer of the Better Place electric car venture.
Reuters and the Associated Press contributed to this report
- Follow Ynetnews on Facebook