Around the time the plane disappeared, the Syrian coastal city of Latakia - near the Russian airbase to which the Il-20 was returning - came under attack from "enemy missiles", and missile defence batteries responded, Syrian state media reported.
CNN quoted the United States official as saying the Syrian army was actually trying to stop a barrage of Israeli missiles, and a second official confirmed that Israel was responsible for the missile strikes on the Syrian positions.
The report said some of the missiles were launched from sea and targeted Latakia, Homs and Hama.
The strikes followed a similar attack on Damascus International Airport late on Saturday, which Syrian state media also blamed on Israel.
Syrian officials have said they do not know who conducted the strike but that missiles "came from the sea".
A USA official said Washington believed the aircraft, which is an Il-20 turbo-prop plane used for electronic reconnaissance, was inadvertently shot down by anti-aircraft artillery operated by Moscow's ally, the Syrian government.
The reconnaissance aircraft was on its way to the Khmeimim airbase and was about 35km from the Syrian coast when it disappeared, the Russian defence ministry said.
While the Russian military said it recorded four F-16 Israeli jets over Syria at the time of the attack on Latakia, the IDF has refused to comment on the report. The Russian Defense Ministry did confirm the aircraft had gone missing in the early hours of September 18, 2018, local time in Syria, but did not say who or what might have been responsible for the crash.
Hassan Ammar/AP PhotoDamascus skies erupt with surface-to-air missile fire as the USA launches an attack on Syria targeting different parts of the Syrian capital, April 14, 2018.
Erdogan said that the deal "will prevent a humanitarian tragedy that could happen as a result of military action" and that the agreement between the two countries would bring "hope to the region".
This would entail a "withdrawal of all radical fighters" from Idlib including the Al Nusra Front and also the withdrawal of heavy weaponry including tanks, multiple launch rocket systems, President Putin said.
Asked whether Syrian President Bashar al-Assad's government agreed with the plan, Russian Defence Minister Sergei Shoigu told reporters in Sochi that "in the coming hours, we will agree with them on all the positions put forth in this document".
It said electricity was later fully restored to Latakia province, a stronghold of Syria's government, after a partial blackout caused by the attack.
The conflict has so far killed more than 350,000 people.
Russian Federation intervened in the Syrian conflict to buttress the Assad regime in 2015.