Story highlights "They don't have a tenable position," Bill Clinton says of Tehran's stance

If Tehran were to get a nuclear weapon, "the retaliation would be incomprehensible," he says

Ahmadinejad says Iran's nuclear program is intended solely for peaceful purposes

Former U.S. President Bill Clinton said Tuesday he does not trust Tehran's assertions that it is not pursuing a nuclear weapon, and he urged the international community to pressure Iran's President Mahmoud Ahmadinejad to allow inspectors to verify his claims.

"What they're really saying is, in spite of the fact that we deny the Holocaust, that we threaten Israel, and we demonize the United States, and we do all this stuff, we want you to trust us," Clinton told CNN's Piers Morgan in an interview to air Tuesday night. "They don't have a tenable position."

If Tehran were to obtain a nuclear weapon, "the retaliation would be incomprehensible," and others in the region would attempt to join the elite club of nuclear powers, Clinton said.

As the number of such weapons grows, the more likely they may be stolen or transferred to terrorists, he said.

"Even if the government didn't directly sanction it, it would not be that much trouble to get a Girl Scout cookie's worth of fissile material which, if put in the same fertilizer bomb Timothy McVeigh used in Oklahoma City, is enough to take out 20%to 25% of Washington D.C. -- Just that little bit."

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In 1996 McVeigh detonated a truck in front of the Alfred P. Murrah Federal Building in Oklahoma City, killing 168 people.

Ahmadinejad has said his country's nuclear program is intended solely for peaceful purposes.

Clinton said the current concern over possible weapons of mass destruction in Iran differs from the situation that occurred in Iraq in 2003, when the United States led an attack on the government of Saddam Hussein after accusing him of harboring such weapons. None was found.

In Iraq, "there was never -- to me -- any credible nuclear intelligence," Clinton said. "This is quite different. They don't even pretend that they don't have centrifuges" of the sort needed to make weapons-grade material.