U.S. and European sanctions targeting the Iranian energy sector go into force within days. Countries that "significantly" cut back on the amount of crude oil imported from Iran are protected from U.S. sanctions.
U.S. Secretary of State Hillary Clinton announced that China and Singapore have met the requirements to get an exemption, bringing the number of exempted countries to 20.
U.S. and European governments have expressed concern Iran is financing a controversial nuclear program with its oil-dependent economy. Clinton said more transparency from Tehran was needed to allay international concerns.
"Failure to do so will result in continuing pressure and isolation from the international community," she said in a statement.
The International Energy Agency states that Iranian crude oil imports have declined to the point that Tehran is out nearly $8 billion in oil revenue per quarter, Clinton said.
A lack of crude oil exports from Libya during last year's war prompted the IEA to call on member states to release oil from strategic petroleum reserves to offset liquidity concerns. Oil markets in 2012 are said to be well supplied as Tehran's consumers had several months to prepare for the decline in Iranian crude.