The British envoy to the OPCW said it had recorded 390 allegations of the use of banned chemicals in Syria since 2014, and that a failure by the OPCW to act risked allowing "further barbaric use of chemical weapons".
"It is our understanding the Russians may have visited the attack site".
The US and France say they have evidence that Mr Assad's forces used poison gas in the attack, but they have not provided any evidence, even after Saturday's missile strikes.
Syrian state media early April 17 said air defenses had shot down several missiles over the central province of Homs.
Within the European Union, which is due to hold an worldwide donor conference for Syria next week, most governments now agree that Assad can not continue as president for peace talks to succeed.
Lavrov also repeated Russia's claim that there was no chemical attack in Douma, asserting that "what did take place was.staged". "The worldwide community is telling Russian Federation that either you make a decision on how you act and when you act, or the rest of us will make a decision in isolating you".
"Chemical weapons were used on Syrian men, women, and children in #Douma".
The OPCW team had been expected to begin their fieldwork on Sunday, but they met with officials at their Damascus hotel instead.
Theresa May is preparing to face questions from MPs about her decision to launch air strikes against military targets in Syria alongside the USA and France.
The missiles that US, French and British warships fired on suspected chemical facilities Saturday constituted the biggest Western attack against the regime in the seven-year war to topple Syrian President Bashar al-Assad.
At least 40 people were killed in the Douma attack.
Tensions have been rising between the U.S. and Russian Federation have been reaching breaking point over conflicts in Syria in recent weeks.
Russian Federation appeared in no mood to extend a hand to the West on Syria however and its ambassador to the UN, Vassily Nebenzia, called the diplomatic push "untimely".
The U.S., United Kingdom and France launched missile strikes targeting three chemical weapons facilities in Syria on Friday.
It was the United States, France and Britain who were "standing in the way" of the investigation by ordering air strikes "in the blink of an eye" before the OPCW team had a chance to do its work.
"I can not be impolite with the heads of other states, but you quoted the leaders of France and the United Kingdom and USA and, frankly speaking, all the evidence they quoted was based on media reports and social media".
"As it is, even with this latest bombing, the West does not have a seat at the table", said the former United States diplomat, now a fellow at the Atlantic Council think-tank.
He responded: "There's one overwhelming reason why this was the right thing to do, and that is to deter the use of chemical weapons, not just by the Assad regime but around the world. Secretary Mnuchin will be announcing those on Monday if he hasn't already, and they will be going directly to any sort of companies that were dealing with equipment related to [Syrian President Bashar] Assad and chemical weapons use".
The U.N. source did not say when the inspectors might visit the site, or whether a planned visit to Douma on Wednesday had been postponed.