The lion's share of Islamic State illegal oil exports is conducted through Turkey and Kurdish areas. Although Washington could curb the illegal traffic, it has chosen to focus on other issues, a former CIA officer told the Sputnik news agency.

"It’s a question of priorities. They have never allocated enough resources to do so. Other goals and missions have been rated as having more urgent calls on intelligence and tactical resources," John Kiriakou, a former Central Intelligence Agency (CIA) counterterrorism officer and US Senate Foreign Relations Committee senior investigator, told Sputnik.

He said Islamic State’s (IS, formerly ISIS/ISIL) oil revenue lifeline could be cut short, if Washington made an effort to do so.

"I do believe that," Kiriakou stressed.

IS makes about $40 million a month on oil sales, raking in close to $500 million a year, a US Treasury Department spokesperson told the news agency earlier this week.

According to Kiriakou, someone on the Turkish side of the border has been making enough money out of it. "There are too many vested interests involved for it to stop. They greased the right people."

ISIS nets $50m a month from smuggled oil, sells crude at super low price up to $10 p barrel https://t.co/lg5OZnSGi8pic.twitter.com/KeODtBdaCM — RT (@RT_com) October 23, 2015

He added: "It’s not the official Turkish government. [It’s] probably corrupt elements of the Turkish military and officials in local and regional governments in southwest Turkey who are involved in this."

The richest oil fields Islamic State can access are south of Irbil in Iraq, and the most likely direction for the extremists to move oil is westwards through Kurdish territory, Kiriakou told Sputnik. Kiriakou says IS' illegal oil trade actually followed the same pattern Saddam Hussein used to defy international economic sanctions. Most of the oil secretly exported from Iraq was moved west through Kurdish territory back then.

"[Selling and transporting oil] through Turkey was the way Saddam Hussein for years beat the sanctions regime imposed on him."

According to Kiriakou, the US should team up with Russia to cut Islamic State’s oil revenue flow.

"We should be working with the Russians to achieve a settlement of the conflict in Syria. We have basically the same aims that they do. Both of us agree that Islamic State is a bad idea and we both want to get rid of it. But we’re not working with them on this," the former CIA officer said.

READ MORE: Obama considers moving US forces deeper into Syria, Iraq – reports

President Barack Obama is reportedly considering several new strategies to target IS in Iraq and Syria. Under consideration is targeting of IS' production and sale of oil on the black market. The US-led military coalition has struck oil refineries controlled by the terrorist group, but officials are looking at using different kinds of weapons to hit the facilities.

A commander of the Free Syrian Army (FSA), Hussam Alawak, told RT earlier this week that a number of FSA officers have recently joined IS.

“They [IS] took lots of people from our army to achieve their goals of expansion, to control oil regions in Syria. They attacked us with very heavy fire in the Al-Bab region [Aleppo province, northern Syria], so we had to tactically retreat to other places."

READ MORE: Islamic State great danger to Iraq's oil industry

The IS advance has already resulted in Iraq losing up to 400,000 barrels of oil daily, the Iraq Oil Ministry spokesman Assem Jihad told Sputnik on Friday. But the Iraqi army had managed to win back almost all the oil deposits captured by the radical Islamic group, according to the spokesman.

"The army and security forces managed to drive IS out of the Saladin Governorate. At the moment, terrorist groups are controlling only a small amount of developed oilfields in the Nineveh Governorate," he said.

Last year, Islamic State seized the biggest oil refinery in Iraq at Baiji, but Baghdad reportedly retook the refinery in October. According to the OPEC Monthly Oil Market Report, Iraq produced 4.14 million barrels per day in September. The country is the cartel's second biggest crude producer after Saudi Arabia.