Syrian government forces used deadly nerve gas in Khan Sheikhun and in three other recent attacks, Human Rights Watch said Monday (May 1st), describing a "clear pattern" of chemical weapons use that could amount to crimes against humanity.

Syrian forces are also stepping up chlorine gas attacks and have begun using surface-fired rockets filled with chlorine in fighting near Damascus, HRW said in a new report.

"The government's use of nerve agents is a deadly escalation -- and a part of a clear pattern," said HRW executive director Kenneth Roth.

"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."

"That is widespread and systematic use of chemical weapons," he said.

The organisation interviewed 60 witnesses and collected photos and videos providing information on the suspected chemical attack in Khan Sheikhun on April 4th and on three other alleged uses of nerve gases in December 2016 and March 2017.

Khan Sheikhun residents said a first bomb believed to be carrying the deadly agent sarin was dropped near the town's central bakery and was followed by three or four high-explosive bombs a few minutes later.

Dozens of photos and videos provided by residents of a crater from the first bomb showed a green-colored metal fragment that HRW said was likely the KhAB-250 bomb.