Syria maintains an ability to deploy chemical weapons, a senior Western diplomat has said, citing intelligence from Britain, France and the United States.
"We have some intelligence showing, that they have not declared everything", a senior diplomat told the Reuters news agency, adding that a "substantial" part of the chemical weapons programme was hidden.
A deadline for Syria to remove all of its chemical weapons will expire on Sunday.
The comments could strengthen allegations Syria's military recently used chlorine gas, a claim dismissed by Russia on Friday.
Syria denies it maintains the capacity to deploy chemical weapons, calling the allegation a US and European attempt to use their "childish" policies to blackmail the government of Bashar al-Assad.
The verification of Syria's poison gas arsenal and its destruction has been overseen by a joint team of the United Nations and the Organisation for the Prohibition of Chemical Weapons (OPCW), the global chemical arms watchdog.
The joint task force said earlier this week that Syria has nearly completed the handover of its chemical weapons stockpile.
Under threat of US airstrikes, Assad agreed with the US and Russia to dispose of his chemical weapons after hundreds of people were killed in a sarin gas attack in August on the outskirts of the capital in August.
Western powers blamed the Syrian armed forces for the attack but the Assad government blamed rebels.
Western diplomats say they have long suspected that Syria did not fully declare its chemical arms programme.
The envoys say they have kept silent on the issue until now to avoid giving Assad an excuse to stop cooperation with inspectors.
Another source cited examples of large batch of a sarin precursor chemical going missing in Syria and unverified claims made by Damascus to have destroyed most of its mustard gas stocks before the UN-OPCW mission arrived in the country.
Concerns are reported to include deadly nerve agent ricin, mustard gas, precursor chemicals used to make sarin, and, more recently, the use of chlorine gas in Syria.
Suspicions that Syria had not fully declared its chemical weapons stocks were deepened when it did not report possessing sarin to the OPCW. The chemical was used in the attack outside Damascus in August.
The senior diplomat said Britain, France and the US had provided information to the OPCW months ago.
He added that the three powers had also provided Assad's staunch ally Russia with the intelligence.