In an interview with Al Jazeera English, Clinton said Iran's leadership has taken actions that show little sign it will respond to President Barack Obama's efforts to engage them diplomatically.
"It's not confidence building, let us say," Clinton said of the recent actions.
"And I think the international community really still wants to engage with Iran, but people are going to now turn to other routes like more pressure, like sanctions to try to change their mind and their behavior," she said.
The new administration has given Iran until the end of the year to seize its offer of engagement.
"President Obama made it absolutely clear with lots of political opposition that if he reached out his hand and if Iran reciprocated, we could talk about anything and everything," the chief US diplomat said.
"Then came the (Iranian) election, then came the crackdown on peaceful dissent, then came demonstrations, and the turmoil inside Iran is continuing until today," she said.
She said Iran has also balked at a US-backed International Atomic Energy Agency (IAEA) proposal for Iran to ship abroad low-grade nuclear fuel so it can be further enriched and returned to refuel a Tehran research reactor.
She said Iran had been forced to come clean on a secret uranium enrichment plant near the holy city of Qom, and noted that Iran has subsequently announced plans for 10 to 20 new nuclear plants.
