The United States announced on Tuesday that it will initially exempt Japan and 10 European nations from its new tough sanctions on Iran, saying these countries have "significantly" reduced their oil imports from the Islamic republic.

Secretary of State Hillary Clinton said she would report to the Congress that sanctions will not apply to the financial institutions in these countries for a renewable period of 180 days.

The 10 European beneficiaries are Belgium, Britain, the Czech Republic, France, Germany, Greece, Italy, the Netherlands, Poland and Spain.

"The actions taken by these countries were not easy," the top US diplomat said in a written statement.

"They had to rethink their energy needs at a critical time for the world economy and quickly begin to find alternatives to Iranian oil, which many had been reliant on for their energy needs," she explained.

President Barack Obama signed a bill at the end of last year, expanding US sanctions against Iran to cover its central bank and financial sector, a move that allows penalties on foreign banks that settle oil imports with the Iranian central bank.

The law, however, offers waivers to firms from countries that significantly reduce their crude imports from Iran. Washington has sent officials to Japan, the Republic of Korea, India and other countries to persuade them to reduce their dependence on Iran's oil.

Clinton noted that the European Union's decision to ban Iranian crude oil imports demonstrates its solidarity and commitment to holding Iran accountable for its failure to comply with its international obligations regarding its disputed nuclear program.

"Japan's significant reductions in crude oil purchases is also especially noteworthy considering the extraordinary energy and other challenges it has faced over the past year," she said.

"We commend these countries for their actions and urge other nations that import oil from Iran to follow their example," she remarked, vowing to increase the pressure on Iran.

Iran insists on the peaceful nature of its nuclear program, while the Western countries have been pressing for a cessation of its uranium enrichment activities to clear their doubt about its intention to make nuclear weapons.