Clinton also said that even if Iran developed such weapons, it would find it tough to use them.
"If they ever use them, they'll be toast," the former U.S. president said in a freewheeling, 90-minute appearance at the annual meeting of the World Economic Forum.
He said that when the Israeli air force took out an Iraq nuclear plant in 1981, it successfully denied the former Iraqi leader, Saddam Hussein, the ability to manufacture nuclear arms.
"I don't know if that option is now available" in Iran, said Clinton.
"That was then. It is much more difficult now," he said, adding that today there are several nuclear powers in the region.
Pakistan and India have nuclear arms, and Israel is widely believed to possess them.
As Clinton spoke, John Bolton, the State Department's top international security official, set off to confer with leaders of Kuwait, Bahrain and the United Arab Emirates on Iran and its nuclear program.
Iran denies it is developing nuclear weapons, and says its nuclear program is only to produce electricity. It wants to enrich uranium, it says, to produce fuel for nuclear power stations.
The United States and several other countries, however, fear Iran is seeking to enrich uranium not to the low level needed to generate power, but to weapons grade, which forms the core of nuclear warheads.
In the past year, Britain, France and Germany have been trying diplomacy to persuade Iran to stop enrichment of uranium and other suspect activities. Those talks, however, have deadlocked with Iran refusal to scrap the programs.
"We ought to give a vigorous push to diplomacy. We must exhaust all reasonable diplomatic activities," said Clinton.
The administration of U.S. President George W. Bush has suggested to take Iran to the U.N. Security Council.
By Naomi Koppel