US President Barack Obama has signed into law new sanctions against Iran intended to impede the development of its nuclear programme.
The measures, which penalise foreign companies that trade with Iran, were overwhelmingly approved by US Congress last week.
Mr Obama said the sanctions would strike "at the heart" of Iran's ability to fund and develop nuclear programmes.
Iran says its nuclear industry is for peaceful purposes; the West disagrees.
The bill targets those firms that supply Iran's Revolutionary Guards or contribute to the country's energy industry, including foreign companies that provide finance, insurance, or shipping services.
Mr Obama said: "We are showing the Iranian government that its actions have consequences, and if it persists, the pressure will continue to mount, and its isolation will continue to deepen.
"There should be no doubt - the United States and the international community are determined to prevent Iran from acquiring nuclear weapons."
Mr Obama noted that Iran had rejected the offer of dialogue and engagement he had made on taking office last year.
"To date, Iran has chosen the path of defiance," he said. "That is why we have steadily built a broader and deeper coalition of nations to pressure the Iranian government."
Tehran denies seeking to acquire nuclear weapons.
Last month, the UN Security Council voted to impose a fourth round of sanctions on Tehran for failing to halt its nuclear enrichment programme.
They include tighter finance curbs and an expanded arms embargo, but not the crippling sanctions the US had wanted.
Iran's President Mahmoud Ahmadinejad dismissed those sanctions as a "used handkerchief" fit for the dustbin.
Three earlier rounds of UN sanctions have blocked trade of "sensitive nuclear material", frozen the financial assets of those involved in Iran's nuclear activities, banned all of Iran's arms exports and encouraged scrutiny of the dealings of Iranian banks.