A MiG-21 combat aircraft flown by a Syrian pilot who defected to Jordan in June was found to have been upgraded back in Syria to carry chemical weapons and to fly without a pilot, the Yisrael Hayom daily reported on Tuesday.

According to the report, U.S. experts who examined the plane believe Russian engineers helped convert the plane and that Syria has more of them in its air force.

On June 21, Syrian pilot Hassan Hamada, who holds a rank equivalent to colonel, took off in his MiG-21 from al-Dumair military airport northeast of Damascus and flew to King Hussein Airbase just across Syria’s southern border with Jordan. Upon landing in Jordan, Hamada removed his rank and requested political asylum.

Syria immediately admitted the pilot had defected and called him a traitor. Suspicion arose when Syria put increasing pressure on Jordan to return the plane. After being examined by Western intelligence agencies, reported Yisrael Hayom, the aircraft was discovered to have the ability to employ chemical weapons and fly without a pilot.

Information about the plane’s additional capability was reportedly passed on to U.S. intelligence agencies, which were said to have studied the information and interviewed Hamada.

After concluding their examinations, U.S. experts said they believe Russian engineers helped convert the MiG-21 to an aircraft with unmanned aerial capability that could be armed with chemical weapons. Officials at the Pentagon believe additional Syrian aircraft underwent the same conversion, although the exact number is unknown.

Reports of the use of chemical weapons by Syrian President Bashar Al-Assad’s troops against the rebels have become more frequent recently.

Al Jazeera reported on Monday that Assad had dropped bombs containing toxic gases, killing six people and blinding others in Homs. Opposition forces released a video showing a victim struggling to breathe after the attack.

Doctors have said that Assad’s forces are probably also using “Agent 15,” which causes paralysis.

Russian Foreign Minister Sergey Lavrov said this past weekend that Syria has acted to consolidate its chemical weapons into “one or two” places to prevent rebels capturing them.

Lavrov said the weapons had previously been “scattered” across the country and they were “under control” for the time being.

The United States said earlier this month that it had intelligence showing Syria was considering using its chemical weapons. U.S. President Barack Obama led international warnings to Assad over the arsenal.