Former U.S. Secretary of Defense William J. Perry said he was concerned that the Islamic State in Iraq and Syria (ISIS) could buy, steal or build a nuclear weapon capable of killing a hundred thousand or more people in a single strike.

And, he said, stopping the flow of oil money to ISIS should be the main, short-term objective of the United States and its allies in the fight against the terrorist organization.

“They have demonstrated their objective is just killing as many Americans as they can, or Europeans as the case may be…and there is no better way of doing that than with nuclear weapons,” Perry said.

Perry made his comments in front of a crowd gathered at Stanford University to celebrate the launch of his new memoir “My Journey at the Nuclear Brink.”

“If they can buy or steal a nuclear bomb, or if they could buy or steal fissile material, they could probably make a bomb – a crude improvised bomb,” he said.

Even a crude nuclear weapon could have an explosive power equivalent to around fifteen thousand tons of TNT – similar to the bomb that destroyed the Japanese city of Hiroshima near the end of World War II.

Perry said there was evidence that Al Qaeda had actively tried to get nuclear weapons, and he said it was likely that ISIS was also pursuing its own nuclear strategy.

“The big difference between ISIS and Al Qaeda in that respect is that ISIS has access to huge amounts of resources through the oil that they now control,” Perry said.

“I believe that our primary objective in dealing with ISIS should be to stop that flow of money, stop the trading they’re doing in oil which is giving them the resources.”

U.S. warplanes reportedly destroyed 116 trucks in Eastern Syria on Monday that American officials said were being used to smuggle crude oil.

U.S. fighter jets dropped leaflets before the attack, warning the drivers to abandon their vehicles, according to a report in The New York Times.

The Russian Air Force also claimed its planes had struck around 500 oil tankers that were carrying oil from Syria to Iraq for processing.

Perry said that combating ISIS over the long run was a “hugely difficult problem” for Western powers.

“To really stop ISIS completely it would be a long and brutal and ugly fighting on the ground, which I don’t believe we’re going to want to do again,” he said.

“What we can do however, a more limited objective is stopping the resources they’re getting, stopping their access to this oil money. And that limits quite a bit what they can do…That can be done I think in more of a targeted and effective way, and without having to put armies on the ground to do it.”