Member states of the world's chemical weapons watchdog on Monday slammed Syria for using poison gas during its civil war and for repeatedly failing to fully explain the scope of its chemical weapons program.

A joint investigation by the United Nations and the Organization for the Prohibition of Chemical Weapons has concluded that Syrian government forces were behind three attacks involving chlorine gas and the Islamic State extremist group was responsible for one involving mustard gas. Syria denies using chemicals as weapons.

"For the first time in the convention's 19-year history, a state party has been found to have violated the treaty's most fundamental tenet," Canadian representative Sabine Noelke said, referring to the Chemical Weapons Convention that banned such weapons and established the OPCW.

The convention and the organization that polices it have been seen as disarmament success stories — the OPCW won the 2013 Nobel Peace Prize — but repeated reports of the use of chemicals as weapons during the Syrian crisis are casting a shadow over that success.

Australia's ambassador to the Netherlands, Brett Mason, told the annual Conference of the States Parties of the OPCW that chemical attacks in Syria are "the most serious violation of the Chemical Weapons Convention in its history" and urged the meeting to take action.

Syria's "flagrant and repeated violations of its basic obligations under our convention cannot be met with silence or excuses by this conference," Mason said.

Syria's Vice Minister of Foreign Affairs Faisal Mekdad rejected the criticism, calling repeated reports of his country's use of chemicals as weapons "part of a coordinated and repeated campaign of lies."

Damascus also faced repeated criticism at Monday's meeting for failing to answer questions about gaps and discrepancies in the declaration it made when it joined the OPCW in 2013 about the extent of its stockpile and chemical weapons program.

"The extent of the gaps in Syria's declaration combined with its willingness to use toxic chemicals as weapons leave substantial grounds for concern that Syria has retained elements of its stockpile and is prepared to use them," said British representative Geoffrey Adams.

The debate was scheduled to continue Tuesday.