The Syrian military has reportedly received six MiG-31M fighter jets from Russia as part of a $1 billion contract signed in 2007, BGN News, an English-language site in Turkey, reported. The fulfillment of the contract, which also included a deal for MiG-29s, is part of Russia’s open support for Syrian President Bashar Assad’s fight against the Islamic State group and Syrian rebel factions that control large portions of country.

The jets reportedly landed at the Mezze air base outside Damascus, the Syrian capital, Sunday evening, the report said. If the report is true, the news will likely raise eyebrows among the U.S. military, which is helping to train anti-Assad forces against whom the aircraft could be used.

Russia has been a strong supporter of the Assad regime for more than four years, shielding it from U.S., European and United Nations pressure over its involvement in the Syrian civil war, which has killed more than 200,000 people and displaced millions more.

Speaking in St. Petersburg, Russia, in June, Russian President Vladimir Putin said he feared "Syria could plunge into the same situation as Libya and Iraq,” and that he was “ready to work with the president [Assad] to ensure a path toward political transition and move away from an armed confrontation. ”

Neither Syria nor Russia has confirmed whether the jets were delivered Sunday.

The 2007 deal wasn’t officially confirmed until 2010, when the head of the Russian company that services the aircraft made the deal public. Syria has an unconfirmed number of MiG-29s that it received from Russia as part of the same deal, IHS Jane's 360, a British publishing company that keeps records on military equipment, reported.

While the Mikoyan MiG-31M was last built in 1994, it is still a very capable platform when fitted with modern components and weapons systems. Russia and Kazakhstan are the only confirmed users of the aircraft.