Syria launched a blistering verbal attack Monday on "Western" countries that have accused it of using chemical weapons in its deadly five-year conflict, dismissing the allegations as "a campaign of lies".

"The multitude of accusations, made in some Western circles without any tangible evidence, as to the responsibility of the Syrian government in cases of use of toxic chemicals are but a part of a coordinated and repeated campaign of lies," Syria's deputy foreign minister Faisal Muqdad said.

He was speaking at the annual conference of countries belonging to the Chemical Weapons Convention, a treaty that compels all member states to help rid the world of toxic arms.

Both the regime of Syrian President Bashar al-Assad and the Islamic State group have been accused of unleashing chemical weapons during the conflict.

More than 300,000 people have been killed since Syria's conflict began with anti-government protests in March 2011.

A panel set up by the UN, known as the Joint Investigative Mechanism, has already determined during a year-long probe that Syrian government forces carried out three chlorine gas attacks on villages in 2014 and 2015.

It was the first time that an international inquiry had pointed the finger of blame at Assad and his forces, after years of denial from Damascus.

The panel consisting of UN and experts from the Organisation for the Prohibition of Chemical Weapons (OPCW) also found that the IS group -- which captured a large swathe of Iraq and Syria in 2014 -- was behind a mustard gas attack in Syria in August 2015.

But Muqdad on Monday disputed the JIM's findings, saying its reports were made on "the basis of inaccurate and unconvincing findings" which "undermines the credibility of the OPCW".

The world should instead be concerned about stopping terror groups like IS from making and using chemical weapons, Muqdad added.

His words were echoed by Syria's main ally Russia, whose deputy trade and industry minister Georgy Kalamanov said the panel's findings remained "unconvincing and sometimes even partial" against Damascus.

Without naming countries, Kalamanov said there were some who "attempt to use the OPCW to overthrow elected governments. That's unacceptable."

- 'Troubling uncertainties' -

Speaking at the start of the five-day conference in The Hague, OPCW chief Ahmet Uzumcu however stressed that "gaps, inconsistencies and discrepancies" remain in Syria's statements about its own chemical weapons programme.

Jacek Bylica, the EU's special envoy for disarmament and non-proliferation said more than three years after Syria joined the convention "many and deeply troubling uncertainties regarding the dismantling of the Syrian chemical programme remain."

"Syria has manifestly failed to declare its full chemical weapons programme," he told delegates.

The EU is "deeply concerned that these chemical weapons might fall into the hands of terrorist groups," Bylica said.

Syria caved to international pressure under a Russia-US brokered deal in September 2013 and agreed to hand over its chemical stockpile to the OPCW for destruction.

It was the first time Syria publicly acknowledged having a chemical arms stockpile and came after a sarin gas attack in August that year on rebel-held areas near Damascus blamed on Assad's regime.

While the OPCW has verified that all of Syria's declared chemical weapons stockpile has been destroyed, it remains concerned by allegations of continuing attacks, many of them said to be from chlorine gas dropped in barrel bombs.

Chlorine, widely used for such things as water purification and fertilisers, is exempt from the convention and does not have to be declared to the OPCW.

Uzumcu told AFP earlier this month that the OPCW is probing more than 20 reports of the alleged use of toxic arms in Syria since August.