The Syrian government and its main ally, Russia, deny that it uses such tactics.
At a news conference held at United Nations headquarters in New York to release the report’s findings, the executive director of Human Rights Watch, Kenneth Roth, ridiculed what he described as “preposterous” assertions by the Syrian and Russian governments denying responsibility.
Mr. Roth said it was time for them “to stop these transparently false diversionary claims and come clean.”
He also said the pattern of attacks as described in the Human Rights Watch report amounted to “a level of culpability and horror that cries out for prosecution.”
On Saturday, an attack on a headquarters of the White Helmets civil defense rescue group in the town of Kafr Zita killed eight of its members, the group and other witnesses say. And medical organizations working in Syria have tallied 10 government attacks in April alone on hospitals and clinics in rebel-held areas, part of a pattern of hundreds of attacks on medical workers and facilities that United Nations investigators have described as war crimes.
Human Rights Watch corroborated claims of two suspected nerve gas attacks on Dec. 12 that initially went relatively unnoticed. This was in part because they took place when the world’s attention was focused on the battle over Aleppo, and in part because of the difficulty of verifying information in the Islamic State-held areas where they occurred.
Medical organizations and social media accounts that day shared images of dead children bearing no visible wounds, as if sleeping, like those killed by a nerve agent in Khan Sheikhoun and in 2013 attacks near Damascus. But because people can be killed for sharing information online from Islamic State-controlled areas, it was difficult to verify them at the time.
Human Rights Watch said its investigators interviewed four residents by telephone and two medics through intermediaries. It said they gave consistent accounts of chemical weapons attacks in two villages in eastern Hama Province, amid clashes between government and Islamic State forces, that killed residents sheltering in caves and in their homes.
The report also provides new details about the Khan Sheikhoun attack, as well as about an intensifying series of recent government bombings and shelling illegally using chlorine gas, with barrels dropped from helicopters and, in a new method, with improvised ground-to-ground missiles.
In those cases, too, the findings coincide with accounts residents and witnesses gave to The Times and with a Times analysis of public information online.
Human Rights Watch corroborated eight chlorine attacks this year, out of a larger number reported by residents. Possession of chlorine, unlike sarin, is not illegal under international law, but its use as a weapon is. The attacks took place in areas where government forces were clashing with rebel forces, near the cities of Damascus and Hama.
The intense battles around Hama led to three attacks, two believed to be with chlorine and one believed to be with a nerve agent, in the two weeks before the Khan Sheikhoun attack. All of them were in al-Lataminah, a town in Hama Province between Khan Sheikhoun and the front line.
On March 25, ordnance crashed through the roof of a clinic that, because of previous attacks, had been reinforced with a metal roof covered with earth. Yellowish gas smelling of bleach filled the facility, killing a doctor, Ali Darwish, as he performed surgery, as well as his patient and another person, according to the Human Rights Watch report and other witnesses. On April 3, munitions with a similar smell again hit the village, injuring at least a dozen.
On March 30, a bomb fell without the usual intense explosion — chemical weapons typically contain a smaller explosive charge, to disperse but not destroy the agent — injuring 169 people, many but not all of them believed to be combatants. They reported symptoms similar to those from a nerve agent, including pupils constricted to pinpoints.
In the Dec. 12 attacks, two villages, Jrouh and al-Salaliyah, were hit, Human Rights Watch said. It quoted a Jrouh resident who said he found his wife, three children, brother, brother’s wife and brother’s three children dead in his basement. He said his neighbors, his uncle and the families of his uncle’s two sons also died.
“Everyone within 100 meters died,” he told the rights group. “There was no one left.” He buried his family and fled, and was interviewed by Human Rights Watch after finding refuge outside Islamic State territory.
Human Rights Watch interviewed 32 residents of Khan Sheikhoun and reviewed available evidence, corroborating previous accounts that one bomb containing a chemical agent fell after a warplane passed over before 7 a.m. on April 4, followed by three or four explosive bombs dropped in a second bombing run.
It found that bomb fragments from the scene of the suspected chemical bomb matched those of a Soviet-made munition that delivers sarin, the KhAB-250.