The Obama administration on Thursday imposed sanctions on a Greek shipping magnate accused of secretly helping Iran sell its petroleum in defiance of international restrictions on Iranian oil exports.

The Treasury Department identified Dimitris Cambis, owner of a Greek shipping company, as a central figure in a scheme that allegedly enabled Iran to disguise its oil to evade international sanctions and an oil embargo.

U.S. officials said Cambis and his businesses purchased at least eight oil tankers on Iran’s behalf, filled them with Iranian oil and sold the crude to international customers who were unaware of its origins. Some of the oil was transferred ship to ship in open seas, in tankers that carry up to $200 million worth of crude at a time, the officials said.

“Today we are lifting the veil on an intricate Iranian scheme that was designed to evade international oil sanctions,” said David S. Cohen, the Treasury Department’s undersecretary for terrorism and financial intelligence.

Cohen said the administration would “continue to expose deceptive Iranian practices and to sanction those individuals and entities who participate in these schemes.”

A phone message requesting comment from Cambis’s Athens office was not returned. In recent interviews, Cambis has acknowledged expanding his oil-shipping business but denied that he was working with Iran.

In an interview with Bloomberg News on Thursday, Cambis dismissed the U.S. allegations as “very bad rumors created by our competitors because we are trying to take market share.”

A senior Obama administration official familiar with the scheme said Cambis essentially went to work for one of Iran’s largest oil companies. The scheme “was designed to hide the fact that the tankers were property of the Iranian government,” said the official, who spoke on the condition of anonymity to discuss the investigation.

“This was a major, not a marginal, step in enforcing our sanctions,” said the official, who added that the punitive measures would not affect the Greek government, which had a “great partnership” with the United States in the international effort to isolate Iran.

Iran, which has seen its oil exports cut in half since the imposition of harsh sanctions last summer, is known to have employed a variety of schemes to circumvent the measures. Last year, international regulators reported that Iran was turning off the navigational transponders on some of its oil tankers at sea, making them harder to track as they search for customers in ports across the Pacific.

The restrictions are part of a series of U.S.-backed measures intended to pressure Iran into agreeing to scale back its nuclear program. So far, Iran has refused to accept limits on its nuclear facilities, insisting that its pursuit of nuclear technology is peaceful.

President Obama on Thursday reiterated his pledge to “keep all options on the table” in seeking to prevent Iran from acquiring nuclear weapons. In an interview with Israel’s Channel 2 television, he said Iran is likely to not possess the capability to make a bomb until 2014.

“We think that it would take over a year or so for Iran to actually develop a nuclear weapon, but obviously we don’t want to cut it too close,” said Obama, who gave the interview ahead of a scheduled visit to Israel next week.

But now, he said, the preferred path is negotiations.

“If we can resolve it diplomatically, that’s a more lasting solution,” he said.