New evidence indicates that the Syrian government used suspected nerve agents in four chemical weapons attacks since December including one in an opposition-held town April 4 that killed nearly 100 people, a human rights group said Monday.

Human Rights Watch said in a report that these attacks “are part of a broader pattern of Syrian government forces’ use of chemical weapons” which could be categorized as crimes against humanity.

The rights group said the April attack in Khan Sheikhoun and the three others using suspected nerve agents all took place in areas where offensives by armed forces fighting the government threatened military air bases.

It said witnesses described symptoms consistent with exposure to nerve agents that they and other local residents experienced after planes attacked northern Hama on March 30 and territory controlled by the Islamic State extremist group in eastern Hama on Dec. 11 and 12.

In the December attacks, Human Rights Watch said four witnesses interviewed by phone and two medical personnel interviewed by text messages through intermediaries gave consistent accounts. An opposition-affiliated activist and local residents provided the names of 64 people who died of chemical exposure, it said.

The group quoted residents, medical personnel and first responders saying the suspected attack on March 30 caused no deaths but injured dozens of people.

In Khan Sheikhoun, Human Rights Watch said 92 people, including 30 children, were identified by residents and activists as victims of deadly chemical exposure. Medical personnel reported that hundreds more were injured, it said.

As part of the evidence showing that attacks have become widespread and systematic, Human Rights Watch identified three different systems being used to deliver chemical weapons:

—Government warplanes dropping bombs with nerve agents.

—Government helicopters dropping chlorine-filled munitions, which has become more systematic.

—Government or pro-government ground forces starting to use improvised ground-launched munitions filled with chlorine.

In at least some attacks, the rights group said, the aim appears to have been to inflict “severe suffering” on the civilian population.

“The government’s recent use of nerve agents is a deadly escalation — and part of a clear pattern,” said Kenneth Roth, executive director of Human Rights Watch.

“In the last six months, the government has used warplanes, helicopters and ground forces to deliver chlorine and sarin in Damascus, Hama, Idlib and Aleppo,” he said. “That’s widespread and systematic use of chemical weapons.”

(AP)