The US Treasury named five men from Iran and elsewhere as well as Dubai-based Belfast General Trading for sanctions violations linked to efforts to support the Islamic republic's acquisition of US currency.
It said Belfast General had converted more than $250 million of Iranian government-held currencies into dollar notes which were then delivered to Tehran.
Also named were an official of already blacklisted Iran-controlled Asia Bank, involved in a separate operation to ship US banknotes to Tehran from Moscow.
Douran Software Technologies, which the Treasury said helps the Iranian government's online monitoring and censorship operations, was also blacklisted.
And Abyssec, an Iranian company which helps Iran's elite Revolutionary Guard in hacking and other online operations, was also named.
"Although we do not support the imposition of any new nuclear-related sanctions while negotiations are ongoing... we have made clear, by word and deed, that we will continue to enforce our existing sanctions," said Treasury Under Secretary for Terrorism and Financial Intelligence David Cohen.
The sanctions forbid Americans from doing business with those on the list and freezes any assets they hold on US territory.
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The Treasury announcement was met with criticism in Tehran.
"We strongly disapprove of this action, which is a blatant violation of goodwill," foreign ministry spokeswoman Marzieh Afkham told reporters.
"It will have no effect whatsoever on the Iranian government meeting its needs. We do not feel duty-bound by illegitimate sanctions," she said.
Washington and its allies have placed a tough regime of sanctions on doing business with Iran's government, its agencies and other institutions to pressure the country to give up its nuclear program.
Iran has been locked in talks with world powers led by the United States since February on a lasting deal to end a decade-long standoff over its nuclear activities.
But they failed to meet a November 24 deadline for a comprehensive agreement that would involve curbs on Iran's enrichment of uranium in exchange for an easing of crippling Western sanctions.
All parties agreed to give themselves seven more months -- until June 30 -- to strike a deal, although they said they hoped to have the broad outlines hammered out by March.
Iran denies it is seeking the bomb and insists its nuclear program is for peaceful energy purposes only.