Hasan Haj Ali, commander of the Free Idlib Army group, said no opposition military positions in the area were targeted on Tuesday.
“Everyone saw the plane while it was bombing with gas,” he said from northwestern Syria.
“Likewise, all the civilians in the area know that there are no military positions there, or places for the manufacture (of weapons). The various factions of the opposition are not capable of producing these substances,” he said.
He called the Russian statement “a lie.”
The Russian Defense Ministry said in a statement earlier: “According to the objective data of the Russian airspace control, Syrian aviation struck a large terrorist warehouse near Khan Sheikhun.”
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 use of such weapons “by terrorists has been repeatedly proved by international organizations as well as official authorities” in Iraq.
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.
Opposition groups led by former Al-Qaeda affiliate Jabhat Fateh Al-Sham vowed revenge for the attack.
The opposition, meanwhile, said the latest comments from Washington softening its line against President Bashar Assad were encouraging him to commit more crimes.
“Until now, this (US) administration has done nothing and adopted an attitude of a spectator, making statements that give the regime an opportunity to commit more crimes,” Abdelhakim Bashar, deputy head of the Syrian National Coalition, told reporters in Istanbul.
The previous administration led by President Barack Obama had always pushed for the ouster of Assad, supporting the opposition members fighting against his forces.
But in an apparent U-turn, the US ambassador to the UN and other top officials in the new administration of President Donald Trump have said ousting Assad is no longer a priority.
“You pick and choose your battles,” US ambassador to the UN Nikki Haley told reporters, echoing comments made by Secretary of State Rex Tillerson on a visit to Turkey last week.
But in a toughening of rhetoric, the White House accused Assad of carrying out a “reprehensible” and “intolerable” chemical attack.
Bashar said the Assad regime posed an even greater danger to security than Daesh and Al-Nusra.
“As long as the regime is in place it won’t be possible to defeat terrorism,” said Bashar, whose group is the main umbrella organization of Syrian opposition organizations.
“Even if Daesh and Al-Nusra are going to be eliminated then this regime would create new terrorist groups to ask the world to choose between it and terrorism,” he said.
Russia said it was pushing on with its support for Assad’s forces.
“Russia and its armed forces are continuing the operation to support the anti-terrorist operation to liberate the country, which the Syrian armed forces are conducting,” Kremlin spokesman Dmitry Peskov told journalists.-ARAB NEWS