The United States imposed sanctions on Thursday on five people and an international network of companies the US Treasury said are involved in the procurement of materials for Iran's nuclear program.
They are the first punitive steps by Washington since Tehran announced earlier this month it would increase its levels of enriched uranium that can be used for bomb fuel.
Tehran announced on July 1 that it had amassed more low-enriched uranium than permitted under its 2015 nuclear deal with world powers, marking its first major step beyond the terms of the pact since the United States withdrew more than a year ago.
"Treasury is taking action to shut down an Iranian nuclear procurement network that leverages Chinese- and Belgium-based front companies to acquire critical nuclear materials and benefit the regime's malign ambitions," Treasury Secretary Steven Mnuchin said in a statement.
"Iran cannot claim benign intent on the world stage while it purchases and stockpiled products for centrifuges," he added.