There is mounting speculation that the Russian Su-25 fighter jet downed in Idlib, north-west Syria was shot down by a US-made missile.

The Russian Defense Ministry said that the Su-25 jet was attacked from the ground by rebels possibly using a man-portable air-defense system (MANPADS) in the designated ceasefire zone and Russian lawmakers have called for a thorough investigation on the origins of the man-portable missile launcher.

The US military has denied sending anti-aircraft weapons to the militias it backs in Syria.

RT reports:”Certainly, we will investigate, including a great many things: from the type of the MANPADS [man-portable air defense system] to the circumstances of the Su-25 downing,” Frants Klintsevich, First Deputy chair of Russia’s Federal Council Defense Committee, told Interfax.

He hopes that evidence will be available due to the “abundance of UAVs and space surveillance in the area.”

As far as military losses go, “the loss of one aircraft is nothing, but politically it has great significance and far-reaching consequences,” Klintsevich said. Other lawmakers are concerned about how the militants could have acquired the anti-aircraft weapon. “We have information that the MANPADS used to bring down our jet was brought into Syria from a neighboring country several days ago,” MP Dmitry Sablin, coordinator of the Russia-Syria parliamentary friendship group, told Interfax. “Countries from whose territory weapons arrive, that are then used against Russian servicemen, must understand that whis will not go unpunished,” he added.

Deputy Head of the State Duma Defense Committee, Yury Shvytkin, told RIA he is inclined to believe that the “MANPADS’ origins are linked with Western countries.”

A Pentagon spokesperson, interviewed by TASS, has already denied the US sent anti-aircraft weapons to Syria. However, the 2017 defense spending bill, signed under Barack Obama, opened the door for the supply of such weapons to Syrian rebels – a fact that Moscow sounded the alarm over when the bill was inked.

However, it envisages careful vetting of the recipients and extensive paperwork, and the Pentagon has previously said that no such deliveries have actually been made.