Russia said Tuesday that its scientific analysis of a suspected chemical weapons attack in Syria on March 19 showed it probably had been carried out by insurgents using sarin nerve gas of “cottage industry” quality delivered by a crudely made missile. The finding contradicted conclusions presented by Western nations, including the United States, that the Syrian government had been responsible.

The Russian analysis, announced at a United Nations news conference by its ambassador, Vitaly I. Churkin, was based on what he said was evidence collected directly by Russian forensics specialists who had been permitted by the Syrian government to visit the site, Khan al-Assal in northern Syria. Mr. Churkin said he had informed Ban Ki-moon, the United Nations secretary general, who has been unsuccessfully pressing the Syrian authorities to permit his own team of chemical weapons experts to visit the country.

Russia is the Syrian government’s most powerful ally and has repeatedly blocked Western-led efforts at the United Nations Security Council to intervene in the Syrian conflict, suspecting that such a step would lead to possible military action that would remove President Bashar al-Assad by force. The Russians have said Mr. Assad and his antagonists should negotiate an end to the conflict, which began as a peaceful uprising against Mr. Assad in March 2011 and has evolved into a civil war.

The Khan al-Assal attack, in which at least 26 people died, has been the focus of a propaganda battle between Mr. Assad’s supporters and opponents, who have accused each other of using chemical munitions.