A former top official of the CIA has revealed the reason why President Barack Obama has not ordered the bombing of oil wells in Iraq and Syria seized by the Islamic State (ISIS)—the primary source of the terrorist group's wealth that finances its operations.

Speaking at PBS "Charlie Rose" TV talk show on Tuesday, former CIA acting director Michael Morell said the reason Obama is not bombing those oil wells is because he is afraid of causing collateral environmental damage, according to WND.

"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," he was quoted by The Hill as saying.

Morell served as the deputy director of the CIA and then as acting director of the agency on two occasions, between 2011 and 2013.

He acknowledged that although he does not "sit in the room anymore" on strategies dealing with ISIS, he's aware that the environmental impact of bombing oil wells was a major concern for Obama.

"There seemed to have been 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," he said.

Instead, the U.S. Air Force has concentrated on bombing oil trucks, Morell said.

The ISIS has accumulated millions of dollars in its war chest from revenues derived from the oil it produces in the wells under its control in Iraq and Syria. Reports are unclear on where ISIS oil exports pass and what groups or countries buy them. Following the downing of a Russian jet fighter by the Turkish air force on Tuesday, Russian President Vladimir Putin reportedly pointed at Turkey as the country where ISIS trades its crude oil.

Republican presidential frontrunner Donald Trump and other Republican primary candidates have been unanimous in calling for a strike on ISIS' oil reserves and other financial resources.

Trump said any plan to defeat the ISIS will hinge on cutting off the revenue stream it gets from oil.

"I'm looking to take the oil. I want to take the oil. I want the oil," Trump said on ABC's "This Week" recently.

"We have to stop the source of money, and the source of money is oil," the businessman added. "We're going to have to take away the energy, the fuel, the money from ISIS."