The United States has imposed fresh sanctions on Iran’s energy sector and some financial companies.
President Obama said the existing sanctions had been made more difficult to evade “by making sanctionable the purchase or acquisition of Iranian petrochemical products".
Iran has been trying to get around sanctions, including by using gold instead of cash as payment.
Mr Obama added that the US remains committed to find a diplomatic solution but that it is Iran’s responsibility to meet international obligations.
He said that firms dealing with the National Iranian Oil Company, the Naftiran Intertrade Company or the Central Bank of Iran, or firms that help Iran buy US dollars or precious metals will be subject to the sanctions.
The new sanctions mean that China's Bank of Kunlun and Iraq's Elaf Islamic Bank will be cut off from the US financial system, as they are seen to have assisted Iran to evade sanctions.
"If the Iranian government continues its defiance, there should be no doubt that the United States and our partners will continue to impose increasing consequences," he said.
