Sanctions halved Iran's oil exports in 2012 by more than 1 million barrels a day, about the amount that oil production grew in the United States during that time, and Washington has been at pains to keep up the pressure.
"We will continue to expose deceptive Iranian practices, and to sanction those individuals and entities who participate in these schemes," Cohen said.
The targeted network bought and operated eight tankers, each able to carry roughly $200 million of oil per shipment, the Treasury Department said.
"These operations are conducted through a series of ship-to-ship transfers in an attempt to mask the fact that the true origin of the oil is from Iran and to introduce it into the global market as if it were non-Iranian oil," Treasury said.
U.S. officials said the sanctions were not aimed in any way at the Greek government, other Greek shippers or the Greek shipping industry in general.
"As it becomes increasingly desperate for foreign exchange, Iran is using increasingly sophisticated techniques to keep its oil flowing," said Mark Dubowitz, with Washington-based Foundation for Defense of Democracies, who has advised U.S. President Barack Obama's administration and U.S. lawmakers on sanctions.
"For every target sanctioned, and covert activity revealed, there will be scores that go uncovered."