"They have consistently tried to avoid being held accountable," Clinton told reporters before leaving on a trip to Latin America.
The U.S. suspects Iran is enriching uranium to build a nuclear warhead. Tehran denies this and insists on its right to a peaceful nuclear power program.
The U.S. hopes to bring a fourth sanctions resolution to a U.N. Security Council vote this week.
On Sunday, Clinton said, "I fully expect Iran to pull some stunt in the next couple of days because they know that sanctions are on the way."
She added, "I think we will see something coming up in the next 24 to 48 hours where Iran says, 'Wait a minute, wait a minute, look at what we're going to do now."
Iran is just one of the issues Clinton will discuss with Latin American leaders during this week's trip. She is expected to press Brazil, which opposes U.N. penalties against Iran, to switch sides.
Brazil, an elected member of the U.N. Security Council, recently worked with Turkey to broker an agreement with Iran that would avert fresh penalties.