GENEVA (Reuters) - Government forces have used chemical weapons more than two dozen times during Syria’s civil war, including in April’s deadly attack on Khan Sheikhoun, U.N. war crimes investigators said on Wednesday.

A civil defence member breathes through an oxygen mask, after what rescue workers described as a suspected gas attack in the town of Khan Sheikhoun in rebel-held Idlib, Syria April 4, 2017. REUTERS/Ammar Abdullah

A government warplane dropped sarin on the town in Idlib province, killing more than 80 civilians, the U.N. Commission of Inquiry on Syria said, in the most conclusive findings to date from investigations into that chemical weapon attack.

The panel also said U.S. air strikes on a mosque in Al-Jina in rural Aleppo in March that killed 38 people, including children, failed to take precautions in violation of international law, but did not constitute a war crime.

The weapons used on Khan Sheikhoun were previously identified as containing sarin, an odorless nerve agent. But that conclusion, reached by a fact-finding mission of the Organization for the Prohibition of Chemical Weapons (OPCW), did not say who was responsible.

“Government forces continued the pattern of using chemical weapons against civilians in opposition-held areas. In the gravest incident, the Syrian air force used sarin in Khan Sheikhoun, Idlib, killing dozens, the majority of whom were women and children,” the U.N. report said, declaring the attack a war crime.

Commission chairman Paulo Pinheiro told a news conference: “Not having access did not prevent us from establishing facts or reasonable grounds to believe what happened during the attack and establishing who is responsible.”

In their 14th report since 2011, U.N. investigators said they had in all documented 33 chemical weapons attacks to date.

Twenty-seven were by the government of President Bashar al-Assad, including seven between March 1 to July 7. Perpetrators had not been identified yet in six attacks, they said.

The Assad government has repeatedly denied using chemical weapons. It said its strikes in Khan Sheikhoun hit a weapons depot belonging to rebel forces, a claim “excluded” by Pinheiro.

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That attack led U.S. President Donald Trump to launch the first U.S. air strikes on a Syrian air base.

A separate joint inquiry by the United Nations and OPCW aims to report by October on who was to blame for Khan Sheikhoun.

The U.N. investigators interviewed 43 witnesses, victims, and first responders linked to the attack. Satellite imagery, photos of bomb remnants and early warning reports were used.

‘GRAVELY CONCERNED’ ABOUT COALITION STRIKES

The independent investigators said they were gravely concerned about the impact of international coalition strikes on civilians.

“We continue to investigate coalition air strikes carried out to expel ISIS from Raqqa resulting in an increasing number of civilian casualties,” Pinheiro said.

The report said: “In al-Jina, Aleppo, forces of the United States of America failed to take all feasible precautions to protect civilians and civilian objects when attacking a mosque, in violation of international humanitarian law.”

A U.S. military investigator said in June the air strike was a valid and legal attack on a meeting of al Qaeda fighters.

“However, the commission however has not found any evidence that such an al Qaeda meeting was taking place,” Pinheiro said.

The American F-15s hit the building adjacent to the prayer hall with 10 bombs, followed by a Reaper drone that fired two Hellfire missiles at people fleeing, the U.N. report said.

“Most of the residents of al-Jina, relatives of victims and first responders interviewed by the Commission stated on that on the evening in question, a religious gathering was being hosted in the mosque’s service building. This was a regular occurrence.”