“In at least some of the attacks, the intention appears to have been to inflict severe suffering on the civilian population, which would amount to crimes against humanity,” the group said in its report, which was based on social media information and interviews with victims, chemical weapons experts and others with knowledge of recent events in Syria.

If substantiated, the repeated attacks would provide new credence to claims that the government of Syrian President Bashar al-Assad has maintained a secret chemical arsenal in the wake of a 2013 disarmament deal and has used it against his adversaries. They would also raise the specter of additional U.S. military action against the Assad government, if it again uses chemical weapons.

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The decision last month to send a barrage of Tomahawk missiles to strike a Syrian air base in the wake of the Khan Sheikhoun attack was a signal of Trump’s willingness to use military force in new ways, in this case to enforce a red line that officials said was ignored by the Obama administration.

In its report, Human Rights Watch documented 12 apparent chemical attacks since mid-December 2016, four of them involving sarin or another unidentified nerve agent. In addition to the Khan Sheikhoun incident, local reports suggest that nerve-agent attacks took place on March 30 and twice on Dec. 12, all of them in Hama province. Close to 70 people were reported killed in those three attacks, all of which occurred in rebel-held areas where government air bases were under threat.

The December attacks allegedly happened in areas under control of the Islamic State, making them harder to document, Human Rights Watch said.

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The Syrian government has rejected accusations it uses chemical weapons and has suggested that the deaths in Khan Sheikhoun were caused by an airstrike that inadvertently struck a rebel chemical munitions depot.

Executive Director Kenneth Roth, speaking at a news conference in New York, said that such a scenario was not plausible in repeated incidents across Syria. Western intelligence and military officials have said there is no evidence that Assad’s opposition possesses chemical weapons.

“This pattern of the Syrian government using nerve agents makes the Syrian and Russian cover story preposterous,” Roth said. Russia is a powerful ally for Assad, whose own military has been weakened by six years of fighting.

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The report also provides details and witness accounts from the attack in Khan Sheikhoun, which Human Rights Watch said killed at least 90 people, 30 of them children. According to interviews with those involved, the effects of the deadly chemicals began to be felt after a series of airstrikes in the early morning of April 4, at least one of them falling near the town’s central bakery. Residents and first responders were among those who exhibited symptoms consistent with sarin exposure.

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“It was like Judgment Day — people were collapsing everywhere,” one resident said in an interview.

Medical analysis conducted in Turkey said the victims of the attacks appeared to have been exposed to sarin.

Human Rights Watch said witness accounts, social media reporting and analysis by foreign governments suggested that the weapon used was one of two Soviet-made bombs, the KhAB-250 or the KhAB-500, designed to deliver sarin.

The report also alleges that the Syrian government’s pattern of dropping chlorine-filled munitions from helicopters, which dates at least to 2014, has become “more systematic.” Pro-government troops have also resumed the use of ground-based attacks using chlorine, it said.