The US has exempted seven countries from economic sanctions in return for cutting imports of Iranian oil.
US Secretary of State Hillary Clinton said waivers were granted to India, South Korea, Malaysia, South Africa, Sri Lanka, Taiwan and Turkey.
Under a US law signed in December, countries have until 28 June to greatly reduce oil imports from Iran or be cut off from the US financial system.
The aim is to pile pressure on Iran to stop its uranium enrichment programme.
Washington and its allies believe Tehran is secretly trying to make nuclear weapons, a claim strongly denied by the Iranians.
Mrs Clinton said the latest exemptions proved that sanctions were working.
"By reducing Iran's oil sales, we are sending a decisive message to Iran's leaders: Until they take concrete actions to satisfy the concerns of the international community, they will continue to face increasing isolation and pressure," she said in a statement.
Pressure on China
In March the US granted exceptions to Japan and 10 EU countries for cutting their imports of Iranian petroleum.
Correspondents say the latest move puts pressure on China, which is Iran's biggest oil customer, to also cut its imports.
US officials have said that Washington remains in talks with Beijing over the issue, AFP news agency reports.
The US says Iranian oil exports have fallen from about 2.5 million barrels a day last year to between 1.2 and 1.8 million barrels a day.
Last week, the UN's nuclear watchdog said talks in Vienna aimed at getting greater access to Iran's nuclear sites had ended without progress.
Further talks between Iran and six powers - Britain, the US, China, Russia, France and Germany - are due to be held in Moscow next week.
Tehran insists its nuclear programme is for peaceful means.