The international body charged with establishing who is responsible for chemical attacks in Syria says it has identified at least five potential cases for investigation — four where chlorine gas was allegedly used against rebel-held areas and one where insurgents allegedly used mustard gas against a rebel stronghold.

The first report by the body established by the U.N. and the Organization for the Prohibition of Chemical Weapons cites two other cases likely to be investigated.

The so-called Joint Investigative Mechanism, or JIM, said in the report circulated Friday that it would start the next phase in March including detailed analysis of the cases, field visits and witness interviews.

Rebels have accused the Syrian government of dropping barrel bombs with chlorine gas — an allegation denied by President Bashar Assad's government.