The Islamic State group may in possession of up to three warplanes, with former military pilots training its fighters to use the jets in battle, it has been claimed.
According to the Syrian Observatory for Human Rights (SOHR), the militant group has Russian-made MiG 21 and 23 warplanes.
The Observatory said the militants are being trained at the al Jarrah airbase on the eastern side of Aleppo.
Rami Abdulrahman, who runs the SOHR, said officers from the Iraqi army who had defected to Islamic State (IS) had "overseen the training of some militants in order to be able to lead these aircraft".
He said local activist had contacted his organisation having spotted the planes flying low over the area. "It is worth mentioning that it is not the first time that the people witness an aircraft flying at a low altitude after taking off from al Jarrah airbase," Rahman said, adding that the jets were purportedly stolen by militants who had occupied airports and airbases affiliated to the Syrian government.
The SOHR said it could not confirm if the Islamic State had any access to missiles.
"We're not aware of (Islamic State) conducting any flight operations in Syria or elsewhere," US Central Command spokesman Colonel Patrick Ryder said.
"We continue to keep a close eye on (Islamic State) activity in Syria and Iraq and will continue to conduct strikes against their equipment, facilities, fighters and centers of gravity, wherever they may be."
US Central Command spokesman Colonel Patrick Ryder told Reuters that the US was "not aware of [Islamic State] conducting any flight operations in Syria or elsewhere."
"We continue to keep a close eye on their activity in Syria and Iraq and will continue to conduct strikes against their equipment, facilities, fighters and centers of gravity, wherever they may be."
Separately, UK authorities have charged four men with terror offences, including taking an oath of allegiance to IS.
The Metropolitan Police force said the men, Tarik Hassane, Suhaib Majeed, Nyall Hamlett and Momen Motasim, who were arrested in London over the past two weeks, were charged with preparing acts of terrorism between July and October, including procuring a Baikal handgun and ammunition and using Google Street View to conduct reconnaissance on a police station and an army barracks in west London.
A fifth man, Nathan Cuffy was charged with a firearms offence. All five are set to appear at Westminster Magistrates' Court on Friday.