Despite a U.S.-Russia negotiated agreement for the Syrian government to surrender its chemical weapons, Russia did not budge Tuesday from its assertion that the Aug. 21 sarin attack near Damascus was probably carried out by rebels.

Foreign Minister Sergei Lavrov said, after a meeting here with his French counterpart, Laurent Fabius, “We have very serious reasons to believe that this was an act of provocation.” He said the ability of the U.N. Security Council to direct an “unbiased, impartial and professional” investigation of the attack will be an “acid test.”

His comments came a day after U.N. inspectors said they had found “clear and convincing evidence” that large quantities of the nerve gas sarin were used last month in Syria.

The inspection report presented to the U.N. Security Council on Monday does not assess blame for the attack. But the report’s underlying evidence, including the trajectory of ­poison-filled rockets, was cited by the United States and its Western allies as proof of the Syrian government’s responsibility.

On Tuesday, Fabius strongly disagreed with Lavrov’s contention. The work carried out by U.N. inspectors proves that the regime of Syrian President Bashar al-Assad is responsible for the attack, he said.

Lavrov said the discovery of Cyrillic lettering on one of the rockets that delivered the gas was not significant. He suggested that other nations, including some in the West, have been counterfeiting old Soviet weaponry, and he called on them to stop doing so.

The Soviets are widely believed to have sponsored Syria’s original chemical weapons program.

Later Tuesday, Human Rights Watch said that it had tracked the trajectory points released in the U.N. report and that they indicate the Republican Guard 104th Brigade base in Damascus as a potential launch site. The base is located in the foothills of the Qudsiya mountains, next to the Syrian Center for Scientific Research Study, long the subject of Western sanctions because of its alleged role in Syria’s chemical weapons program.

The rockets have a range that allows for them to have been fired from the Republican Guard base, but they could also have been launched from other points on the plotted trajectory, said Peter Bouckaert, emergencies director at Human Rights Watch.

Their disagreement aside, the two foreign ministers were in accord over the need to work out a Security Council resolution on the disposal of Syria’s chemical arms, and on the need to find a way to start a new round of peace talks along the lines suggested by Russia and the United States this past spring.

But Lavrov made it clear that Russia will veto any resolution that countenances the use of force in case of Syrian violations. Such a resolution would fall under Chapter 7 of the U.N. Charter, which allows the international body to approve military action — something Russia has adamantly refused to allow regarding Syria.

Colum Lynch at the United Nations, Karen DeYoung in Washington and Loveday Morris in Beirut contributed to this report.