The United States says 11 countries have “significantly reduced” their purchases of Iranian oil and will not be subject to U.S. sanctions intended to pressure Iran over its controversial nuclear program.

In a statement Tuesday, Secretary of State Hillary Clinton commended the countries – mostly European – and urged other importers of Iranian oil to follow suit. She offered special praise for Japan, saying it had cut imports while facing “extraordinary” energy challenges.

Japan's Chief Cabinet Secretary Osamu Fujimura said Wednesday his country is ready to go even further.

“We have told the U.S. side that the rate of decrease will accelerate and Iranian crude imports will be reduced substantially from now on, even more so than in the previous five years.”

He said Japan told the United States the rate of cuts will accelerate.

The other countries whose banks will not be affected by the sanctions are Belgium, the Czech Republic, France, Germany, Greece, Italy, the Netherlands, Poland, Spain and Britain. The exemptions last for a renewable period of 180 days.

U.S. Treasury Secretary Timothy Geithner told a House committee Tuesday that European support has been “critical” and that the United States is encouraged by broader cooperation with Iran's trading partners.

“But now, we're going to keep looking at ways we can bring more pressure to bear. And we're going to keep looking at what's the most effective balance of pressure we can bring to bear, but I think we're making really substantial progress. And our hope is, of course, that it will alter Iran's calculations about their interests and pursuing a nuclear capability.”

The sanctions, which go into effect June 28, bar financial institutions from accessing the U.S. financial system if they conduct transactions with Iran's Central Bank.

If the country that has primary jurisdiction over a bank significantly cuts its oil imports from Iran, President Barack Obama can grant an exemption. He must also determine if the price and supply of oil from countries other than Iran allows importers to make those cuts.

The president can waive the sanctions in the name of national security.

Iran denies allegations it is attempting to develop atomic weapons and says its nuclear activities are purely for power generation and medical research purposes.