The US has given China and Singapore exemptions from sanctions for buying Iranian oil.
US Secretary of State Hillary Clinton said that both countries had earned the exemptions by cutting imports.
All of the twenty biggest importers of Iranian oil have been granted reprieves from the sanctions, before the deadline on June 28.
The sanctions would have seen the concerned nations cut off from the US financial system.
"Their cumulative actions are a clear demonstration to Iran's government that Iran's continued violation of its international nuclear obligations carries an enormous economic cost," Clinton said in a statement.
“We have been clear all along that there is a path for Iran to fully re-join the global economy. Iran’s leaders have the opportunity to address international concerns by engaging seriously and substantively in negotiations with the P5+1.
"I urge Iran to demonstrate its willingness to take concrete steps toward resolving the nuclear issue during the expert-level talks scheduled in Istanbul on July 3.
"Failure to do so will result in continuing pressure and isolation from the international community.”