The British embassy to the Netherlands on Monday said Russian Federation and Syria had not yet allowed members of a fact-finding mission to enter the site of a chemical attack in Syria.
The missiles that US, French and British warships fired on suspected chemical facilities Saturday were the biggest Western attack against the regime in the seven-year war.
Assad denounced a "campaign of deceit and lies at the (United Nations) Security Council" after a push by Moscow on Saturday to condemn the strikes fell far short.
Syria and its Russian ally are "waging a single battle - not only against terrorism, but also to protect global law based on the respect of the sovereignty of states and the will of their people", Assad's office quoted him saying during a meeting with Russian politicians.
"OPCW arrived in Damascus on Saturday".
Talks were held behind closed doors at the Netherlands seat of the Nobel Peace prize-winning Organisation for the Prohibition of Chemical Weapons, two days after its mission arrived in Damascus.
The April 7 strike on Douma, in which most experts say chlorine as well as an agent such as sarin were used, killed at least 40 people, according to local medics.
Faisal Mekdad, Syria's deputy foreign minister, said on Monday that government officials have met with the delegation, which has been in Damascus for three days, a number of times to discuss cooperation.
The inspectors will face a hard task, with all key players having pre-empted their findings, including Western powers, which justified the strikes by claiming they already had proof such weapons were used.
Its team on the ground also faced the risk of arriving too late to a site where much of the evidence may have always been removed or tampered with.
Damascus and Moscow have vehemently denied that any chemical weapons were used and alleged instead that grim videos showing civilians foaming at the mouth after the attack were staged.
Kenneth Ward, the U.S. ambassador to the OPCW, raised these fears on Monday, drawing a denial from the Russian foreign minster, Sergei Lavrov, who told the BBC: "I can guarantee that Russia has not tampered with the site". Their arrival coincided with a Syrian military announcement that it had "purified" the region of eastern Ghouta, of which Douma is a part, after a two-month campaign that has killed almost 2,000 civilians, following years of siege.
The bombings, hailed by U.S. President Donald Trump as a success but denounced by Damascus and its allies as an act of aggression, marked the biggest intervention by Western countries against Assad and his ally Russian Federation, whose foreign minister Sergei Lavrov called them "unacceptable and lawless".
As the on-the-ground investigation fails to get under way, the fallout from the US-led response continues to reverberate, with French President Emmanuel Macron claiming to have persuaded President Donald Trump to keep his troops in Syria.
British Tornado and Typhoon warplanes, American B-1 bombers and French Rafale jets also took part in the strikes.
His ambassador to the UN, Nikki Haley, announced yesterday the United States was "locked and loaded" should the need arise.
Western leaders have called for a fresh diplomatic push, aiming to end a conflict that has killed more than 350,000 people and displaced half of Syria's population.
Regime and allied forces are now expected to train their sights on southern districts of Damascus that are still held by the Islamic State jihadist group. They blame Assad's government for the attack.