The United States said Wednesday it was renewing the exemptions for Japan and some European Union countries from tough US sanctions imposed on nations buying oil from Iran.

A total of 20 countries have "continued to significantly reduce the volume of their crude oil purchases from Iran," US Secretary of State John Kerry said in a statement.

Under a law meant to pressure the Iranian leadership over its contested nuclear program, the United States bars banks from nations that buy Iran's key money-maker from doing business in the world's largest economy.

Kerry announced he was extending for another 180 days exemptions first granted on March 20 last year to Belgium, Britain, the Czech Republic, France, Germany, Greece, Italy, Japan, the Netherlands, Poland and Spain.

The European Union completely banned oil imports from Iran from July 1, reducing its purchases of Iranian products to zero. Financial institutions in 10 EU countries which still have non-petroleum transactions with the Central Bank of Iran have therefore been granted a further renewable 180-day exemption from sanctions.

Japan, a close US ally, has also reduced imports despite energy shortfalls in the wake of the tsunami and nuclear incident two years ago, allowing it an exemption.

"The United States and the international community remain committed to maintaining pressure on the Iranian regime until it fully addresses concerns about its nuclear program," Kerry said.

"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."

Iran says that its uranium enrichment is for peaceful purposes, but Western nations and Israel accuse Tehran of covertly seeking to develop an atomic bomb.