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.