Justin Bronk, an air combat expert at the Royal United Services Institute, said he doubted Russia's claims, and that the remarks were "probably just posturing in this case to try and embarrass the USA".
While weapons shipments into Syria are not uncommon, these flights involving Iran have caught the attention of United States intelligence because they occurred in the days after the April 13 U.S. airstrikes on Assad regime targets. "With such a large number of weapons being fired and anti-aircraft fire being directed at them, you would expect, at the very least, a small proportion of them not to get through", he said.
"A few years ago at the request of our partners, we decided not to supply S-300s to Syria", he said.
Mueller says access across conflict lines also "remains extremely constricted".
On Tuesday, Russia's Kommersant newspaper, citing sources, said that S-300s would be given - not sold - to Syria and that issues surrounding the transfer "have practically been resolved". Syria, which destroyed all of its chemical stockpiles in 2013 and 2014 under worldwide supervision, has denied the claims, while Moscow provided evidence that the Douma attack was actually fabricated. The organization cannot determine which party was responsible for chemical use, and can only say whether they were or were not used.
"The briefing is organized in support of the OPCW fact-finding mission now deployed in Syria to investigate the incident", the press release said.
On April 14, US President Donald Trump along with his French counterpart, Emmanuel Macron, and British Prime Minister Theresa May authorized a joint missile attack against alleged chemical weapons manufacturing sites inside Syria. Their entry into the town was delayed by security fears.
Immediately after the attacks, many people who worked at these destroyed facilities and just bystanders without any protective equipment visited them.
The watchdog is not mandated to apportion blame for the attack.
Britain announced $630 million for 2018 and another $418 million for 2019, while Germany said it would donate $1.218 billion and the European Union pledged some $791.8 million.
Earlier, a chemical attack happened in Douma in East Ghouta on April 7, when 70 people died as a result.
Responding to Russian plans to visit its headquarters in The Hague with the Syrians, the OPCW said it has advised the Russian delegation that "these persons should be first interviewed" by the inspectors who are in Damascus.
The OPCW, which has sent a team of inspectors to Syria to investigate the alleged attack, had no immediate comment.
The development is part of an ongoing clash of narratives between the West on one side and Syria and Russian Federation on the other concerning the suspected attack. It follows a rebel fighter whose wife and child were killed in the chemical attack, and is forced himself to take up arms against an attack from government forces.
Last week, Lavrov told the BBC that Russian Federation is no longer bound by its pledge not to supply S-300s to Syria after the April 13 strikes by the U.S., Britain and France against three suspected chemical weapons sites in Syria.
Russia's Defense Ministry has also accused Britain of direct involvement in staging video images of alleged victims. Britain vehemently denied the Russian accusation.
But as Ron Paul argued in a recent column, even if the Syrian army did carry out the gas attack in Douma, evidence of this still wouldn't justify the US, UK and France bombing targets inside a foreign country.
Russian S-400 Triumph/SA-21 Growler medium-range and long-range surface-to-air missile systems are transported during the Victory Day parade at Red Square, in Moscow, on May 9, 2015.
Top officials in Russian Federation said it might reconsider its pledge a decade ago not to provide Syria with the S-300 system.