Islamic State (Isis) is takings its first steps towards building an air force by training pilots to fly captured fighter planes, according to a group monitoring the conflict in Syria.

Isis is using lots of tanks, armoured personnel carriers, artillery and Jeeps taken from the Syrian and Iraqi armies but this is the first report that it has planes in the air.

Isis, which took the US by surprise this year with its rapid territorial expansion in Syria and Iraq, has three Russian-built MiG jets, according to the Syrian Observatory for Human Rights (SOHR), which appears to have a good network of observers on the ground and has often proved reliable in the past.

Rami Abdulrahman, who runs the British-based group, said Isis has trainers who had gained experience in the Iraqi air force under former president Saddam Hussein.

Abdulrahman cited witnesses who had seen the planes flying low over Aleppo, in rebel-held northern Syria.

If Isis was able to train pilots, they could mount 9/11-style suicide attacks on key installations in Damascus or Baghdad.

Defence analysts said that while this is feasible, they were extremely sceptical. “There is a small chance but not a big chance,” said Afzal Ashraf, a former RAF captain, counter-terrorism specialist and consultant at the London-based Royal United Services Institute(Rusi).

It was feasible to train pilots for small aircraft but it would be a struggle to do the same with an advanced fighter jet. Such jets would require frequent maintenance as well as a good navigation system and air traffic control, Ashraf said.

Such planes could be easily picked off by US, UK, Turkish or even Syrian planes or air defences. The UK is flying reconnaissance and bombing missions only over Iraq while the US is engaged in both Iraq and Syria.

In February last year, the SOHR reported the capture of the al-Jarrah military airport near Aleppo, which had been held by Assad’s forces. The MiG fighters were said to have been taken at the time.

The monitoring group reported witnesses saying the planes flying low over Aleppo recently appeared to be MiG 21s or MiG 23s and had taken off from and returned to the nearby al-Jarrah base. It added that training courses are taking place at the base.

The SOHR said it did not know whether the planes were armed with missiles.

Isis has taken swathes of Sunni-dominated territory in Iraq and is on the outskirts of Baghdad.

Raffaello Pantucci, a counter-terrorist specialist at Rusi, said he had seen footage of captured planes and questioned their viability. “They were covered in bird shit,” he said.

Pantucci said they would pose no problem for the sophisticated air defence capabilities of the US and its Nato ally Turkey, and even to a lesser extent those of Syria’s President Bashar-al-Assad.

Ashraf said of the former Iraqi air force pilots reported to have joined Isis that it would be hard for them to keep up with the latest advances. “Flying is a very perishable skill,” he said.

He predicted they were almost certain to crash. “You have to be able to fly and to take off and to navigate. If you do not know what you are doing, you are likely to stall it,” Ashraf said.