US President Barack Obama has ordered new economic sanctions against Iran's energy sector and some financial firms.
The move is to stop Iran from setting up payment mechanisms for the purchase of oil to get round existing sanctions.
Mr Obama said the US remained committed to reaching a diplomatic solution on Iran, but that the onus was on Tehran to meet its international obligations.
The sanctions come amid ongoing concern over Iran's nuclear programme, which Tehran denies is to develop weapons.
Existing sanctions on Iran's oil industry had been expanded "by making sanctionable the purchase or acquisition of Iranian petrochemical products", Mr Obama said in a statement.
Measures would be taken against firms that have dealings with the National Iranian Oil Company, the Naftiran Intertrade Company or the Central Bank of Iran, or that help Iran buy US dollars or precious metals, he added.
The new sanctions also targeted China's Bank of Kunlun and Iraq's Elaf Islamic Bank as institutions that "knowingly enable financial transactions for designated Iranian banks".
The move was a commitment to hold Iran "accountable for its actions", Mr Obama said.
He said the sanctions made it clear that the US would expose any financial institution that assisted "the increasingly desperate Iranian regime" to access the international financial system.
"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 added.
Mr Obama has been criticised by Republican presidential challenger Mitt Romney for failing to act strongly enough to stop Iran from gaining nuclear weapons.