"We consider it unacceptable to analyse events from a distance," Foreign Minister Sergei Lavrov said at a press conference in Moscow with his counterparts from Syria and Iran.
Mr Lavrov said Mr Assad's opponents had "in essence" given guarantees for the Organisation for the Prohibition of Chemical Weapons to visit the location where at least 87 people died, but the watchdog was refusing to send them.
"They say still that it is not very safe, but they cannot put forward convincing arguments," Russia's top diplomat said.
Russia has rejected accusations from the West that its Assad's forces were responsible for a chemical attack and has lashed out at the US for its cruise missile strikes last week against a Syrian air base.
The OPCW said Thursday that a fact-finding mission was analysing samples gathered from "various sources" and that allegations a chemical attack took place in the Syrian rebel-held town of Khan Sheikhun were "credible".
Mr Lavrov said Russia, Iran and Syria have demanded a "thorough, objective and unbiased investigation" under the auspices of the OPCW, insisting it must use "independent experts", including from Moscow.
Russia and Iran are the firmest allies of Mr Assad's regime and have deployed forces to the war-torn country to back him in the country's brutal six-year conflict.
Mr Lavrov was meeting with his counterparts from Syria and Iran in a show of support for Damascus after US Secretary of State Rex Tillerson visited Moscow earlier this week.
Mr Lavrov repeated the Kremlin's condemnation of the US missile strike in Syria and said Washington was seeking "excuses for regime change".
"These attempts will not succeed, this will not happen," he said.
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