Bylica delivered the damning statement on behalf of the EU on the opening day of the annual meeting of member states of the Organization for the Prohibition of Chemical Weapons in The Hague.
Syria`s Deputy Foreign Minister Faysal Mekdad rejected the claims, saying that only a few "technical elements" remained to be ironed out.
Syria declared an arsenal of 1,300 tons of chemical weapons when it joined the OPCW in 2013 under pressure from the international community following a deadly chemical attack on a Damascus neighborhood.
More than 99 percent of those weapons have been destroyed, but questions remain about whether Syria declared all of its stockpiles.
The OPCW has a special team reviewing Syria`s declarations amid fears that if the nation still has chemical weapons or production capabilities they could fall into the hands of insurgents fighting in the country`s civil war, including the Islamic State group.
Bylica said that during behind-closed-doors meetings this year member states were shown "alarming findings" of the declaration review team, saying that in five of the team`s reports on Syria, "more new questions arose than found satisfactory answers." The reports remain confidential.
A recent report by a separate international fact-finding team in Syria revealed that a "non-state actor" used the blistering agent sulfur mustard, an outlawed chemical weapon, in an attack near the northern Syrian town of Aleppo in August, likely killing an infant. There have also been allegations of Syrian government forces using the widely available chemical chlorine as a weapon.
Mekdad denied those allegations.
"We wish here to state categorically that we have never used chlorine or any other toxic chemicals during any incidents or any other operations in the Syrian Arab Republic since the beginning of the crisis and up to this very day," he said.
The United Nations and OPCW have established a joint investigation team to apportion blame for such attacks. OPCW Director-General Ahmet Uzumcu said Monday that the team is expected to issue its first report in February.
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