WASHINGTON - U.S. Secretary of State Hillary Clinton on Thursday announced Washington's decision to exempt China and Singapore from sanctions over imports of Iranian oil, citing their "significant" reduction of purchases from the Islamic republic.

"Today I have made the determination that two additional countries, China and Singapore, have significantly reduced their volume of crude oil purchases from Iran," the top U.S. envoy said in a written statement.

She said as a result, financial institutions in the two Asian countries will be eligible for exemptions from U.S. sanctions for a renewable period of 180 days.

Under the National Defense Authorization Act (NDAA) signed by U. S. President Barack Obama late last year, foreign financial institutions, whose governments still purchase Iranian crude oil as of June 28, will be denied access to the U.S. financial market.

"Today marks an important milestone in the implementation of the NDAA and U.S. sanctions toward Iran," Clinton remarked. " Following the president's determinations on March 30 and June 11 on the availability of non-Iranian supplies of oil, as of today, any foreign financial institution based in a country that has not received an NDAA exception is subject to U.S. sanctions if it knowingly conducts a significant transaction with the Central Bank of Iran for the sale or purchase of petroleum or petroleum products to or from Iran."

A total of 20 economies, including Japan, India, Malaysia, the Republic of Korea, South Africa, Sri Lanka, Turkey, Taiwan and 10 European countries, have got the U.S. waiver.

"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, pointing to the sharp drop in the republic's oil exports from some 2.5 million barrels per day (bpd) in 2011 to about 1.5 million bpd at present, with lost revenues of 8 billion U.S. dollars every quarter in real terms.

The UN Security Council imposed four rounds of sanctions on Iran between 2006 and 2010 over its refusal to halt its nuclear enrichment program, which Western countries suspect could be used to develop nuclear weapons. Despite Iran's insistence that its nuclear program is for civilian use only, the United States and European Union (EU) have imposed extra sanctions of their own and continued expansion of the sanctions.

The EU has decided to press ahead with a full oil embargo on Iran starting on July 1, though the Islamic republic, following three rounds of talks over its uranium enrichment program since mid-April, has agreed to meet again with the U.S., Britain, France, Russia, China and Germany, the so-called P5+1, in Istanbul in July at the expert level.

"We have been clear all along that there is a path for Iran to fully rejoin the global economy," Clinton said. "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," she noted, adding "Failure to do so will result in continuing pressure and isolation from the international community."