The Trump administration has condemned Iran's government for cracking down on protests across the country and warned that the United States has "ample authorities" to impose sanctions in response.
At the same time, the US imposed new sanctions on five Iranian entities over their involvement in developing ballistic missiles.
Those sanctions were unrelated to the ongoing protests. But Treasury Secretary Steven Mnuchin said more sanctions "targeting human rights abuses are coming," adding that the US would call out Iran's economic mismanagement - a key complaint of the protesters.
At the State Department, spokeswoman Heather Nauert said Iran's government had imprisoned more than 1,000 people and was killing "those who are brave enough" to protest in the streets.
She accused Iran's government of limiting the flow of information, restricting speech and trying to prevent the rest of the world from witnessing the repression.
Ms Nauert said the US supports Iranians' "legitimate aspirations" and calls on Iran's government to ease controls on information and allow peaceful demonstration.
"To the regime's victims, we say: You will not be forgotten," she said.
The fresh expression of support for the protesters came as the US looked for additional ways to embolden and assist those demonstrating against corruption, economic problems and mismanagement by Iran's government.
The new sanctions target subsidiaries of Shahid Bakeri Industrial Group, part of the Iranian Defence Ministry. Shahid is already under US sanctions. The new designations ensure its subsidiaries are punished too.
The US has called on Iran to stop blocking social media sites and has sought through the media and other channels to signal to Iranians that the world is watching closely and lending moral support.
Iran, in return, has tried to portray the protests as fomented by the US, rather than the spontaneous result of homegrown frustrations.