Former CIA Deputy Director Michael Morell says that the Obama administration did not bomb oil wells under control of the Islamic State group because it was concerned about the environmental impact.

"We didn't go after oil wells, actually hitting oil wells that ISIS controls, because we didn't want to do environmental damage, and we didn't want to destroy that infrastructure," Morell told PBS' "Charlie Rose" Tuesday evening, reported The Daily Caller.

Morell explained that the White House was also concerned that destroying the oil infrastructure could be detrimental to the Syrian people in the long run once ISIS is defeated.

Prior to the Nov. 13 terrorist attacks in Paris, "there seemed to be a judgment that ... look, we don't want to destroy these oil tankers because that's infrastructure that's going to be necessary to support the people when ISIS isn't there anymore, and it's going to create environmental damage. And we didn't go after oil wells - actually hitting oil wells that ISIS controls because we didn't want to do environmental damage and we didn't want to destroy that infrastructure, right," Morell said.

However, after the Paris attacks, the U.S. began bombing oil trucks in an attempt to cut off the terrorist group's main source of revenue, noted The Hill. The U.S. hit more than 100 oil tanker trucks on Nov. 16 alone, and more strikes this week took out an additional 238 trucks, according to The Daily Mail.

"So now we're hitting oil trucks," Morell said. "And maybe you get to the point where you say, we also have to hit oil wells."

"So those are the kind of tough decisions you have to make," he added.

The extremist group is thought to make nearly $500 million annually from extracting and exporting oil from the vast swaths of Syria and Iraq under its control, according to The Washington Times.

While the Obama administration has showed restraint in directly striking the group's oil reserves, Republican presidential candidates like Donald Trump have advocated the exact opposite, proposing forcefully attack the group's oil fields and other sources of revenue, reported CNBC.

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