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 are said to have defected.
However, IS have three planes which they captured earlier on the ground in Aleppo and Raqqa, the BBC's Sally Nabil in Cairo reports.
Meanwhile, the Iraqi government says its troops have 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.
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.