China and India on Wednesday were again exempted from US sanctions aimed at choking off Iran's oil exports after significantly cutting back on purchases of crude from the Islamic republic.

"The United States and the international community stand shoulder to shoulder in maintaining pressure on the Iranian regime until it fully addresses concerns about its nuclear program," US Secretary of State John Kerry said.

He said in a statement that Beijing and New Delhi were among 20 countries which "have again qualified for an exception to sanctions" due to "significant reductions in the volume of their crude oil purchases from Iran or for reducing those purchases to zero and remaining there."

Other countries also granted exemptions from any sanctions were Malaysia, South Korea, Singapore, South Africa, Sri Lanka, Turkey and Taiwan, with the rest coming from Europe.

The US administration ruled that the economies concerned had taken steps to reduce imports of Iranian oil, so their financial institutions would not face measures under US law aimed at curbing Tehran's nuclear program.

Under US sanctions legislation the administration is bound by law to reconfirm the exemptions every six months.

"The message to the Iranian regime from the international community is clear: take concrete actions to satisfy the concerns of the international community, or face increasing isolation and pressure," Kerry said.

Under a 2011 law approved last year, the United States threatened to penalize foreign financial institutions over transactions with Iran's central bank, which handles sales of the country's key export.

There have been 10 failed meetings between the International Atomic Energy Agency (IAEA) and Iran over the past 18 months, as negotiators aim to get Tehran to rein in its suspect nuclear program.

On Wednesday the six world powers engaged in the stalled talks said it was "essential and urgent for Iran to engage with the agency on the substance of its concerns."

Western countries and Israel have accused Iran of seeking to develop atomic weapons, while Tehran insists its nuclear program is entirely peaceful.