Iraqi pilots who have joined Islamic State are training its members in Syria to fly three captured fighter jets, according to a UK-based activist group that monitors the conflict.
The Syrian Observatory for Human Rights (SOHR) said witnesses had seen the planes being flown around a military airport in Aleppo.
Meanwhile, Iraqi forces have launched an attack on IS militants near Tikrit.
The city was among the areas in Syria and Iraq seized by IS this year.
Rami Abdul Rahman, head of the SOHR, said IS was using Iraqi officers who were pilots under ex-Iraqi President Saddam Hussein to train fighters in Syria.
"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.
It is not known how many Iraqi pilots have defected.
Witnesses told the SOHR the planes appeared to be MiG-21 or MiG-23 models.
The BBC's Sally Nabil in Baghdad says IS has three planes which it captured earlier from the Syrian military in Aleppo and Raqqa.
US Central Command spokesman Col Patrick Ryder told Reuters the Pentagon was "not aware" of IS conducting any flight operations in Syria or elsewhere.
Aleppo became a key battleground in the fight between Syrian rebels, which now include IS, and government forces after the uprising against President Bashar al-Assad in 2011.
Separately, the Iraqi government said its troops had gained ground to the north and west of Tikrit and cut an important IS supply route.
Correspondents say that past efforts by the government to regain territory in the area have ended in failure.
Meanwhile Kurdish forces, backed by US-led air strikes, are continuing to fight militants in the northern Syrian town of Kobane.
US-led warplanes struck IS positions on Friday, taking advantage of new co-ordination with the town's Kurdish defenders.
The Pentagon confirmed there were six air strikes near Kobane, saying initial reports indicated they successfully "struck three IS buildings, destroyed two IS fighting positions, suppressed three IS fighting positions and destroyed two IS vehicles".
There was another air strike on an IS oil facility near Shadadi.
The US military also said its "partner nation military forces" had conducted two air strikes in Iraq, near Baiji.
On Thursday, Kurdish commander Baharin Kandal told the BBC that the militants had been driven out of most of Kobane.
Our correspondent says refugees watching their town from a hilltop nearby agree that there are only two or three neighbourhoods that are still the scene of fighting.
The battle for Kobane, which is also known as Ayn al-Arab, is regarded as a major test of whether the US-led coalition's air campaign can push back IS.
Activists say more than 600 people have been killed since the jihadist group launched its assault on the mainly Kurdish town a month ago.
More than 160,000 people have fled in the face of the IS advance.
Capturing the town would give the group unbroken control of a long stretch of the Syrian-Turkish border.
IS fighters have gained a reputation for brutal tactics, including mass killings and beheadings of soldiers and journalists.