McCain opened a Senate hearing Wednesday by saying that Iran will get the bomb unless the United States acts more boldly.
Speaking figuratively, the Arizona Republican says the U.S. keeps pointing a loaded gun at Iran but failing to "pull the trigger."
Military and intelligence officials testifying before Congress said Iran's accelerated nuclear program could produce one weapon in roughly a year, if the country decided to go that route.
But Gen. James Cartwright predicts it would still take longer than that to make the bomb usable.
Cartwright said that, historically, it takes a country three to five years to make such a leap. Cartwright is the second highest-ranking U.S. military officer.
The timeline he cited Wednesday could be shortened if Iran pursued ways to deliver a weapon at the same time as it worked to build a bomb.
The U.S. government has prepared a new, classified, assessment of Iranian nuclear ability and intent but has not released it yet.
The State Department's No. 3 official said the United States is working as fast as it can to win new international sanctions on Iran.
William Burns predicted that a resolution will emerge from the United Nations Security Council within weeks. And Burns called the case for new penalties urgent, saying he expects China will agree to some form of sanctions.
The U.S., a traditional stumbling block on the United Nations Security Council regarding Iran, during a U.S.-hosted summit on nuclear proliferation this week.
Iran has denied it is seeking a nuclear weapon and maintains its ambitions are strictly energy-related. Tehran has also scoffed at U.S. threats to increase pressure. As the Obama administration welcomed nations to the nuclear summit Monday, Iran was busy celebrating "National Nuclear Day," reported CBS News correspondent Elizabeth Palmer from Tehran.
Despite the rhetoric brushing aside the prospect of sanctions, the head of the country's nuclear program, Ali Akbar Salehi,in a rare interview with CBS News.
"Of course sanctions will affect us," said Salehi, who earned his Ph.D. at MIT in Boston."But it only will delay our projects. It will not stop our projects."
"They will hurt," he added. "Then we will have to come up with our own manufacturing systems."
Iran already uses homemade technology to enrich uranium in one plant, and recently announced it would build six more.