The US has exempted Japan and 10 EU nations from sanctions imposed on countries which continue to buy oil from Iran.
The sanctions, imposed by Congress at the end of last year, are designed to deter countries from importing Iranian oil.
In a statement released on Thursday, US Secretary of State Hillary Clinton said the exemptions were granted because the countries had taken difficult steps to reduce their reliance on Iranian oil.
"They had to rethink their energy needs at a critical time for the world economy and quickly begin to find alternatives to Iranian oil which many had been reliant on for their energy needs."
The exemptions mean Japan and the EU countries, including the UK, France, Germany, Greece and Italy, can continue to buy some oil from Iran without being penalised by the US.
The sanctions stipulate that the United States must cut off the US bank account of any foreign financial institution that completes petroleum-related transactions with Iran's central bank, unless its country is exempt.
Other major buyers of Iranian oil, including China, India and South Korea are not exempt from the sanctions and are under continued pressure to reduce imports.
