The Assad Government has repeatedly used chemical weapons to consolidate its power in Syria, according to new evidence that has "decimated" the Russian "cover story" for the deadly April 4 attack on Khan Sheikhoun.
The world was shocked last month when it was revealed that Syria had used a nerve agent against its own citizens in the rebel-held Idlib province town, killing 92 people, including at least 30 children, according to news.com.au.
The attack prompted US President Donald Trump to launch 59 Tomahawk missiles at a Syrian airfield.
But new evidence released on Monday by Human Rights Watch shows that Bashar al-Assad's government used nerve agents three times leading up to the April atrocity, which show "widespread and systematic attacks on civilians [that] could constitute crimes against humanity".
Russia, which supports the Assad regime, justified the Khan Sheikhoun attack by saying a Syrian bomb had inadvertently hit a rebel cache of nerve gases or pesticides.
But Human Rights Watch executive director Ken Roth says new evidence renders that explanation "utterly implausible".
The organisation has revealed that the Syrian military used chemical weapons just five days before the April 4 attack at a town 15km southwest of Khan Sheikhoun.
This attack did not kill anyone but injured dozens of civilians and rebel insurgents.
Syrian forces also dropped nerve agents twice in mid-December near the eastern Hama towns of Jrouh and Al-Salaliyah, an area controlled by Islamic State.
A rebel-affiliated activist and local residents say 64 people died from chemical exposure in those attacks.
"This pattern of the Syrian government using nerve agents makes the Syria-Russia cover story preposterous," Roth said at a briefing attended by news.com.au in New York.
"There's no way that on four different occasions, in four different places, Syrian forces just happened to hit caches of nerve agents, even ignoring the fact that there's no evidence that such caches ever existed.
"It's time for Moscow and Damascus to stop these transparently false, diversionary claims and to come clean about this grotesque breach of international law."
The organisation has also confirmed multiple cases of government forces using helicopters and ground-launched rockets to deploy chlorine-filled munitions "pervasively".
Roth said the report showed chemical weapons had become a "central part of its military strategy".
"And as so often happens with Syria's military strategy under the Assad Government, these attacks are directed not only at military forces, but very often also at the civilian population and civilian institutions," he said.
"As such, these widespread and systematic attacks on civilians could constitute crimes against humanity.
"That is a level of culpability and horror that cries out for prosecution."
Roth said the pattern of attacks suggested that the Syrian Government had kept sarin gas or a similar nerve agent after the August 2013 Ghouta sarin gas attack.
International condemnation following the attack forced the country to agree to hand over all of its chemical weapons to United Nations inspectors.
"Assad apologists have also asked: why would the Syrian government risk using nerve agents when the war seemed to be going its way? But all four of these attacks were in areas where opposition or Isis forces were launching an offensive that threatened government military air bases," Roth said.
"Short on ground troops and already having gotten away with using chlorine as a chemical weapon, the Assad Government decided to deploy a nerve agent."
Human Rights Watch's report also reveals new detail about what weapon was used in the April 4 attack.
Its analysis concludes it was most likely a KhAB-500, a Soviet-made air-dropped chemical bomb, specifically designed to deliver deadly sarin gas.
Local residents told the organisation that a warplane flew over Khan Sheikhoun twice about 6.45am.
It dropped a bomb near the town's central bakery. Witnesses did not hear an explosion but did see smoke and gas.
This was followed by another flyover, which resulted in three or four highly explosive bombs being dropped on the town.
First responders told Human Rights Watch that hundreds of people in the area exhibited signs of nerve-agent exposure, including frothing at the mouth, pinpoint pupils, constricted breathing and trembling.
Roth said the Russian Government, led by President Vladimir Putin, was culpable for its role in covering up the truth.
"The Russians seem determined to try to poke a hole in any evidence that's put forward," he said.
"But the primary cover story they've presented so far is decimated by this report.
"The Assad Government is utterly dependent on Russian military support. Both Tehran and Moscow have the capacity to stop these chemical attacks. The fact that they are choosing not to, that they are continuing to provide military support depside ample evidence of chemical weapon use makes them complicit in these war crimes as well."
Human Rights Watch has condemned Russia and China for using its UN Security Council veto to stop Syria from going before the International Criminal Court.
It has called on Syria to fully co-operate with investigators from the UN and the Organisation for the Prohibition of Chemical Weapons, and proposed that the Security Council impose a travel ban and asset freeze on the Syrian government.