"According to the objective data of the Russian airspace control, Syrian aviation struck a large terrorist warehouse near Khan Sheikhun," the Russian defence ministry said in a statement.
It housed "a warehouse making bombs, with toxic substances", said the ministry, without stating if the strike was voluntary or deliberate.
"The arsenal of chemical weapons" was destined for fighters in Iraq, the ministry said, adding that the information was "completely reliable and objective."
The statement did not specify whether the Syrian regime knew there were chemical weapons there and pointed the finger at "terrorists" who they say hold the toxic arms.
More From This Section
At least 72 civilians including 20 children died in a suspected chemical attack yesterday in the rebel-held Syrian town of Khan Sheikhun in Idlib province in the northwest.
The Syrian Observatory for Human Rights monitoring group said the attack also left dozens suffering respiratory problems and symptoms including vomiting, fainting and foaming at the mouth.
The army denied any involvement in a statement blaming "terrorist groups" for using "chemical and toxic substances".
Rebel groups led by former Al-Qaeda affiliate Fateh al-Sham Front vowed revenge for the attack.
The incident brought swift international condemnation, with the United States, France and Britain presenting a draft resolution to the UN Security Council demanding a full investigation.