The Syrian opposition has asserted that the Islamic State in Iraq and the Levant formed an air combat fleet.

The Syrian Observatory for Human Rights said ISIL captured fixed- and rotary-wing aircraft from Iraq and Syria in mid-2014. The London-based monitoring group said ISIL acquired at least three Russian-origin fighter-jets as well as an unspecified number of combat helicopters.

“The combat aircraft are believed to be the MiG-21,” Syrian Observatory said.

In a statement on Oct. 17, Syrian Observatory said defectors from the Iraq Air Force was training ISIL to fly the aircraft. The opposition group, regarded as authoritative, said training was taking place in an air base in the northern province of Aleppo.

“Officers from the dissolved Iraqi military, who are also members in the Islamic State, have overseen the training of some militants to fly these aircraft,” Syrian Observatory said.

So far, all of ISIL’s air fleet stemmed from platforms seized in its offensive in the Aleppo and Raqa provinces in the summer of 2014. At least two Syrian Air Force bases were captured and said to have housed fighter-jets, attack helicopters and air defense batteries.

Syrian Observatory cited testimony from Syrians who live near the Jarah air base in Aleppo. The witnesses were said to have seen at least one MiG take off from Jarah and fly at low altitude.

“This is not the first time that an aircraft was reported flying at a low altitude after taking off from Al Jarah air base,” Syrian Observatory said. “There has not been confirmation whether ISIL has missiles for these aircraft.”

Syrian Observatory director Rami Abdul Rahman said the Iraqi trainers had been air force pilots under the regime of the late President Saddam Hussein. Abdul Rahman said ISIL was overseeing numerous flights from Jarah, located east of Aleppo city and 70 kilometers south of Turkey.

“They flew many times from that airport,” Abdul Rahman said.

The United States did not confirm the report by Syrian Observatory, and officials said they doubted whether ISIL had the spare parts to maintain its new air fleet. Over the last month, the U.S. Air Force has led a coalition in strikes on ISIL facilities in Iraq and Syria.

“We don’t have any operational reporting of ISIL flying jets in support of ISIL’s activity on the ground,” the U.S. military’s Central Command chief, Gen. Lloyd Austin, said. “And to the degree that pilots may have defected and joined the ranks of ISIL, I don’t have any information on that either.”