US Secretary of State Hillary Clinton said she thinks Iran will "pull some stunt" in the next few days because it expects further United Nations sanctions over its nuclear programme.
"They have consistently tried to avoid being held accountable," Ms Clinton said before leaving on a trip to Latin America.
The US suspects Iran is enriching uranium to build a nuclear warhead. Tehran denies this and insists on its right to a peaceful nuclear power programme.
The US hopes to bring a fourth sanctions resolution to a UN Security Council vote this week.
"I fully expect Iran to pull some stunt in the next couple of days because they know that sanctions are on the way," Ms Clinton said.
"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 she will discuss with Latin American leaders during this week's trip. She is expected to press Brazil, which opposes UN penalties against Iran, to switch sides.
Brazil, an elected member of the UN Security Council, recently worked with Turkey to broker an agreement with Iran aimed at averting fresh penalties.