“Our assessment of the program remains the same... We are very concerned about where they might be headed. An Iran with a nuclear weapon is unacceptable,” said George Little, acting assistant secretary of defense for public affairs.

“The United States has made that point crystal clear that all options are on the table to prevent Iran from developing that capability,” Little said.

“We don't know, at this stage, whether they have made the decision to proceed with the development of a nuclear weapon. It would be unacceptable if they do move forward,” he said.

The Pentagon spokesman said he believes sanctions are the most effective way to pressure Iran.

The West accuses Iran of pursuing a secret nuclear weapons program but Tehran insists it needs nuclear power solely to generate electricity.