WASHINGTON The White House moved to tighten sanctions against Iran on Monday because of the country's suspect nuclear program, freezing all property of the Central Bank of Iran, other Iranian financial institutions and the Iranian government in the United States.

The new restrictions also raised new warnings to financial institutions in other nations that they could face big penalties in the United States if they do business with Iran's central bank.

The actions were announced in an executive order signed by President Barack Obama that started the enforcement process for a tough measure he signed into law at the end of 2011. If fully carried out, that measure could isolate Iran's central bank and effectively choke off the sale of Iranian oil by blocking any means of payment. Most of the revenue for oil sales by Iran, one of the world's biggest oil exporters, is processed by its central bank. Obama's actions came against the backdrop of rising tensions between Iran and the West over Tehran's nuclear energy program, which Iran says is peaceful and Western nations say is merely a cover for Iranian efforts to build a nuclear weapon.

Israeli leaders, who regard Iran as their most serious security threat, have dropped hints that they may order a pre-emptive military attack on suspected nuclear targets in Iran if they conclude the sanctions are ineffective. Obama and his aides have sought to restrain Jerusalem, emphasizing in recent days that sanctions and diplomatic pressure are the preferred strategy to dissuade Iran.

"There has been a steady increase in our sanctions activity, and this is part of that escalation," Jay Carney, the White House spokesman, told reporters after the latest steps were announced.

Last week an International Atomic Energy Agency inspection team left Iran after the government refused to allow it to scrutinize elements of the nuclear program that the agency has said could be military in nature. Although the team is returning to Iran this month, Western diplomats have concluded that Iran appears intent on stonewalling.

In a statement, the White House said the executive order "re-emphasizes this administration's message to the government of Iran - it will face ever-increasing economic and diplomatic pressure until it addresses the international community's well-founded and well-documented concerns regarding the nature of its nuclear program."

The timing, a Treasury Department official said, was driven by the need for the administration to lay the groundwork to effectively enforce the new sanctions law, in part by reaching out to allies and other countries to explain how the sanctions will work. Many of these countries buy oil from Iran through its central bank, and their financial institutions could be barred from the U.S. market if they continue to do so.

The administration has been working to convince Iran's oil customers to shift to other suppliers, and countries like Saudi Arabia have pledged to raise production to compensate for any shortages. Some of Iran's biggest customers, however, notably China and India, have signaled their intention to keep purchasing Iranian oil.

Iran's government has denounced all the sanctions as an effort by the U.S. and its allies to bully Iranians into surrendering their legal right to peacefully pursue nuclear energy. Iran has also threatened to retaliate for the sanctions by closing the Strait of Hormuz, a vital Persian Gulf waterway for oil shipments, and by cutting off European buyers of Iran's oil ahead of a planned EU embargo of Iran oil to start on July 1.

Iran's official Islamic Republic News Agency, reporting on an organized rally against the sanctions in Tehran on Monday, said participants chanted "Death to America" and "Death to the Zionist regime" and called for the parliament to "take immediate action in halting the export of oil to European countries."