Russia — which has questioned whether the Douma attack even happened — ridiculed the Western accusation, asserting that the United Nations had exercised its authority to delay the inspectors for security reasons.
The United Nations disputed the Russian explanation, saying it had no security issues and wanted the inspectors to reach the site quickly.
When, or even whether, the inspectors would be allowed unfettered access to the site remained unclear Monday night, despite Russian and Syrian promises of cooperation.
The Douma attack led to airstrikes in Syria over the weekend by the United States and its allies, Britain and France, which said they believed that President Bashar al-Assad’s forces had carried it out.
The Western allies said the airstrikes were aimed at degrading the chemical arms capabilities of Mr. Assad, who promised to purge Syria’s arsenal of chemical weapons after a mass attack in 2013.
The repeated use of chemical weapons in the Syria conflict, a war crime, reflects what many disarmament experts describe as new levels of impunity that threaten respect for international treaties and the rule of law.
Mr. Assad has repeatedly denied using chemical weapons. President Vladimir V. Putin of Russia and his subordinates have called the accusations of Russian complicity a lie that has maligned the Kremlin, fed Western “Russophobia” and contributed to the worst East-West tensions since the Cold War.
Even as the war in Syria exacts a fearful toll on the ground, discussion of the Douma attack, like so much of the international posturing about the war, has been wrapped in a fog of contradiction and confusion. Nations made charges and countercharges, claiming to have damning but secret evidence about each other’s conduct, with Russia in particular spinning an array of theories of varying degrees of plausibility.
Syrian and Russian officials have told the Organization for the Prohibition of Chemical Weapons team “that there were still pending security issues to be worked out before any deployment could take place,” Ahmet Uzumcu, the organization’s director general, told its executive council on Monday. The meeting was held in private, but the organization released the prepared statements of Mr. Uzumcu and some other officials.
Mr. Uzumcu also said the Russians and Syrians had offered the inspectors an opportunity to interview 22 witnesses to the Douma attack. He did not say whether the organization had accepted the proposal.
The British delegation to the organization wrote on Twitter: “Russia & Syria have not yet allowed access to Douma. Unfettered access essential. Russia & Syria must cooperate.” Other Western diplomats also said that Syria and Russia were impeding the team.
The United States ambassador to the organization said at the meeting on Monday that Russia could be trying to conceal evidence of chemical weapons.
“It is our understanding the Russians may have visited the attack site,” the ambassador, Kenneth D. Ward, said. “We are concerned they may have tampered with it with the intent of thwarting the efforts of the O.P.C.W. fact-finding mission to conduct an effective investigation.”
In an interview with the BBC, Sergey V. Lavrov, the Russian foreign minister, said, “I can guarantee that Russia has not tampered with the site.”
Senior Russian diplomats said it was the United Nations, not Syria or Russia, that had prevented inspectors from entering Douma. “The problem was the absence of the U.N. secretariat security department’s approval for O.P.C.W. experts to visit Douma,” Sergei A. Ryabkov, deputy foreign minister of Russia, told reporters, according to the news agency Interfax.
But a spokesman for the United Nations, Stéphane Dujarric, said the United Nations had given the inspectors “all the necessary clearances.”
Mr. Dujarric declined to say whether Secretary General António Guterres would demand that Russia and Syria provide the experts with access to the Douma site. But the spokesman said Mr. Guterres wanted the investigation to move forward “so we can have a full picture of all the facts.”
Even before the O.P.C.W. inspectors arrived in Syria, the Western allies said that they had ample evidence that the Syrian government had dropped a chemical agent on Douma, and that it had used chemical weapons many times during the seven-year civil war.
East-West tensions have escalated since Britain accused Russia of using a powerful nerve agent to poison a Russian former spy living in Britain and his daughter. The Kremlin has denied the accusation, which set off the diplomatic expulsions and a series of economic measures against Russia.
“It is the style of today’s London to blame Moscow for everything and ascribe certain actions to which we bear no relation whatsoever,” Mr. Ryabkov, the deputy foreign minister, said.
Russian and Syrian officials have suggested that a chemical attack never occurred in Douma, or that it was staged by rebel forces or Western powers as an excuse for attacking Syria. Mr. Lavrov accused Britain of playing a part in the supposed ruse, which British officials called outrageous.
The Syrian military, with help from its Russian and Iranian allies, this month retook control of Eastern Ghouta, a suburban area that was the last major rebel-held enclave near Damascus, the capital. Douma was the last part of the region to fall.