The official alleges, with corroboration from other sources, that Russia flew a drone above a hospital as victims of the nerve gas attack started to enter, AP reports.
Hours after the drone flew away from the hospital, a Russian airstrike hit the hospital. The anonymous official claims this was to distract attention away from the use of a chemical weapon.
The allegation continues that the drone's existence "could not have been a coincidence" and was further evidence of the Russians' knowledge.
Syrian and Russian airstrikes have continued in the rebel-held areas of Idlib, regardless of the international backlash against the attack in Khan Sheikhun.
Videos from the past three days show incendiary bombs and what appears to be phosphorus being used in civilian areas of Idlib province.
The incendiary bombings came days after Washington carried out its first military action against Bashar al-Assad's government since the start of Syria's six-year war.
Baghdad appeared to distance itself from the Assad regime on Monday in response to the attack, the first time in the history of Syria's six-year civil war.
The Shia-led Iraqi government issued a carefully worded statement, condemning the Khan Sheikhun attack.
Agencies contributed to this article
