Samples taken by the Organisation for the Prohibition and Chemical Weapons (OPCW) tested positive for chemicals needed to make toxic agents, diplomatic sources told Reuters on Friday.

The government led by the Syrian president Bashar al-Assad agreed to completely eliminate its chemical program in 2013, following a deadly gas attack which many in the international community blamed on the regime forces, and a threat of military intervention by the United States. Last year, the authorities handed over some 1,300 tons of chemical weapons to be destroyed by a joint UN-OPCW mission.

However, the samples which tested positive for precursor substances were taken in December and January, from a military research center not declared to OPCW, according to the unnamed sources.

"This is a pretty strong indication they have been lying about what they did with sarin," one of the diplomats told Reuters.

Traces 'where they were not supposed to be'

In addition, Latvia's permanent representative at OPCW, Maris Klisans, stated to fellow delegates at a closed-door meeting Thursday that the European Union had a number of "concerns" over the issue, adding that "the recent finding … showing traces of precursors of VX and sarin were found on a site where they were not supposed to be, figure high on that list."

"The EU is particularly concerned that, due to the above, Syria may still hold chemical weapons materials or undeclared chemical weapons agents," she said in the statement which was later published on the OPCW's website.

OPCW spokesman Peter Sawczak declined to provide more information, saying he was bound by confidentiality on the matter.

The blame game

Under the conditions put forward by Washington and Moscow two years ago, Damascus has agreed not to use the toxins in combat.

Still, according to the OPCW, chlorine has been used "systematically and repeatedly" even after Syrian government shipped away its massive stockpile. The international watchdog is not mandated to assign blame, however, and the regime has denied using chemical weapons.

On Wednesday, the United States asked the UN Security Council to set up an investigation following reports of chlorine gas attacks in Syria.

"We believe - and it's clear that many Council members agree - that we have got to have a means of establishing who is carrying out these chlorine attacks," US ambassador to the United Nations, Samantha Power, told reporters on Friday.

Britain, France and the United States have accused the government forces of carrying out the chlorine attacks, using barrel bombs thrown from helicopters. At the same time, Russia maintains there is no solid proof that Assad's troops are behind the attacks.

dj/gsw (AFP, Reuters)