The measures are seen as aimed at cutting Iran's access to imports of gasoline and isolating it further from the international financial system. Iran, though a major oil producer, heavily relies on imports of refined oil products such as gasoline.

US President Barack Obama signs the Iran Sanctions Bill in the East Room of the White House in Washington, DC, July 1, 2010.[Xinhua]

In the statement, Obama accused Iran of being the only one signatory to the NPT (Non-Proliferation Treaty) that has been unable to convince the international nuclear watchdog that its nuclear program is for peaceful purposes.

"To date, Iran has chosen the path of defiance. That is why we have steadily built a broader and deeper coalition of nations to pressure the Iranian government," he said.

On June 9, the UN Security Council adopted a resolution, imposing the fourth round of sanctions on Iran since 2006.

On June 16, U.S. Treasury Secretary Timothy Geithner announced that the U.S. government will extend sanctions on more Iranian individuals and entities, including the Post Bank of Iran, Revolutionary Guards commanders, Iranian shipping companies and so on.

The new sanctions imposed by U.S. congress and government go far beyond the UN resolution both in scale and scope.

However, Obama added in the statement that the door to diplomacy is still open, urging Iran to meet its international obligations.

"That is the Iranian government's choice. And it remains our hope that they choose this path, even as we are clear-eyed about the difficult challenges ahead," Obama said.