New evidence shows a "pattern of chemical weapons use", according to a report launched by the rights group at the UN headquarters in New York.

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

“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," Roth said.

According to the group, the "widespread and systematic" nature of the attacks, as well as them being directed against civilian populations might be legally sufficient to define them as crimes against humanity.

The chemical weapons attack in Khan Sheikhun, Idlib killed at least 92 people, including 30 children, according to the group, grabbing the headlines across the world.

However, the 48-page HRW report says there were three other cases where a Syrian military aircraft dropped a nerve agent on Syrian towns -- al-Lataminah on March 30, 2017, and Jrouh and al-Salahiyah on December 12, 2016 -- all located in the Hama governorate.

The report, based on interviews with dozens of witnesses as well as analysis of photo and video evidence, identifies three different delivery methods used by the regime:

- Government warplanes appear to have dropped bombs with nerve agents on at least four occasions since December 12;

- Government helicopter-dropped chlorine-filled munitions have become more systematic;

- Government or pro-government ground forces have started using improvised ground-launched munitions filled with chlorine.

"The United Nations Security Council should immediately adopt a resolution calling on all parties to fully cooperate with investigators from the Organisation for the Prohibition of Chemical Weapons (OPCW) and adopt sanctions against anyone UN investigators find to be responsible for these or past chemical attacks in Syria," the group said.

On April 12, the UN Security Council failed to condemn the Khan Sheikhun attack after opposition from permanent member Russia, the principal backer of the Bashar al-Assad regime.

The OPCW said late last month "incontrovertible evidence" exists that sarin gas was used in the attack, the deadliest of its kind since the Ghouta attack in August 2013 killed more than 1,300 hundred people.

Anadolu Agency