The group, which has seized land in Syria and Iraq, has been flying the planes over the captured al-Jarrah military airport east of Aleppo, said Rami Abdulrahman, who runs the Britain-based Syrian Observatory for Human Rights.

U.S. Central Command said it was not aware of Islamic State flying jets in Syria. "We're not aware of ISIL conducting any flight operations in Syria or elsewhere," said Central Command spokesman Colonel Patrick Ryder, using an acronym for the group.We continue to keep a close eye on ISIL activity in Syria and Iraq and will continue to conduct strikes against their equipment, facilities, fighters and centers of gravity, wherever they may be," Ryder said.

U.S-led forces are bombing Islamic State bases in Syria and Iraq. The group has regularly used weaponry captured from the Syrian and Iraqi armies and has overrun several military bases but this was the first time it had been able to pilot warplanes.

"They have trainers, Iraqi officers who were pilots before for (former Iraqi dictator) Saddam Hussein," Abdulrahman said."People saw the flights, they went up many times from the airport and they are flying in the skies outside the airport and coming back," he said, citing witnesses in northern Aleppo province near the base, which is 70 km (45 miles) south of Turkey.

It was not clear whether the jets were equipped with weaponry or whether the pilots could fly longer distances in the planes, which witnesses said appeared to be MiG 21 or MiG 23 models captured from the Syrian military.

Pro-ISIS Twitter accounts had previously posted pictures of captured jets in other parts of Syria, but the aircraft had appeared unusable, according to analysts and diplomats.

The countryside east of Aleppo city is one of the main bases of ISIS in Syria, where the al Qaeda offshoot controls some region of the country's territory.

The Israeli Air Force has raised its alert level in the north over reports that Iraqi defector pilots are training ISIS air crews to fly missions using captured Syrian MiGs warplanes, Israel’s 0404 News said Friday.

A captured jet at the al-Tabqa air base

After heavy fighting in late August, ISIS militants overran the al-Tabqa air base on August 24th, and captured three Syrian MiG-21 or MiG-23 models, as well as missiles and related gear, according to the London-based Syrian Observatory for Human Rights (SOHR).

At least three planes were shown in a propaganda video by the extremists following the battle for the al-Tabqa Syrian Air Force base.

Militants claimed they killed “about 200” Government prisoners after the battle and the propaganda video showed men being forced to lie face-down and shot.