CIA director John Brennan has said that Islamic State fighters have used chemical weapons and have the capability to make small quantities of chlorine and mustard gas, CBS News reported Thursday.

"We have a number of instances where ISIL has used chemical munitions on the battlefield," Brennan told CBS News, which released excerpts of an interview to air in full on the "60 Minutes" news program on Sunday.

The network added that he told "60 Minutes" the CIA believes that the IS group has the ability to make small amounts of mustard or chlorine gas for weapons.

"There are reports that ISIS has access to chemical precursors and munitions that they can use," Brennan said. 

Brennan also warned of the possibility that the Islamic State group could seek to export the weapons to the West for financial gain.

"I think there's always the potential for that. This is why it's so important to cut off the various transportation routes and smuggling routes that they have used," he said.

When asked if there were "American assets on the ground" searching for possible chemical weapons caches or labs, Brennan replied: "US intelligence is actively involved in being a part of the efforts to destroy ISIL and to get as much insight into what they have on the ground inside of Syria and Iraq."

 'Toxic chemicals in Iraq, Syria'

 The release of the interview excerpts comes two days after similar comments from spy chief James Clapper before a congressional committee.

"ISIL has also used toxic chemicals in Iraq and Syria, including the blister agent sulfur mustard," Clapper, the director of national intelligence, told lawmakers on Tuesday.

He said it was the first time an extremist group had produced and used a chemical warfare agent in an attack since Japan's Aum Supreme Truth cult carried out a deadly sarin attack during rush hour in the Tokyo subway in 1995.

President Bashar al-Assad's regime and rebel forces have accused each other of using chemical agents in the nearly five-year war that has killed more than 250,000 people. 

After an August 2013 sarin attack outside Damascus that much of the international community blamed on Assad's government, the regime agreed to turn over its chemical arsenal.

The Organization for the Prohibition of Chemical Weapons (OPCW) -- which oversaw the dangerous removal and elimination of Syria's avowed stockpile -- now says that declared arsenal has been completely destroyed.

But the global arms watchdog has still warned of the continued use of mustard, sarin and chlorine gas in the conflict, without blaming the regime, the rebels or the IS group for use of the weapons, which are banned under international law.

Last year, officials in the autonomous Iraqi region of Kurdistan said blood tests had shown that IS fighters used mustard agent in an attack on Kurdish peshmerga forces in August.

Thirty-five peshmerga fighters were exposed and some taken abroad for treatment, officials said.

At the time of the attack, The Wall Street Journal cited US officials as saying they believed IS had used mustard agent.