The poison gas attacks in Damascus were launched from a military base controlled by Bashar al-Assad's forces, Human Rights Watch has said, as more evidence emerged to implicate the regime.
Russia, though, insisted the evidence pointed to a rebel attack.
HRW's analysis supports the accusation made by John Kerry, the United States Secretary of State, that the missiles used to deliver sarin nerve gas in the attacks on August 21 were fired from an area of Damascus under Assad's control.
When United Nations investigators published their report on the incident on Tuesday, they were forbidden from naming the perpetrator. But they did reveal the trajectories flown by two of the missiles.
HRW used that data to pinpoint the locations from which the weapons were launched.
"When mapping these trajectories, the presumed flight paths of the rockets converge on a well-known military base of the Republican Guard 104th Brigade."
The base is in an area of northern Damascus held by the regime, close to the presidential palace. The "key details" revealed in the UN report "strongly suggest the Government is to blame", said HRW.
The weapons which appear to have been fired from the Republican Guard base landed in the Damascus suburbs of Moadamiya and Ein Tarma.
The UN inspectors examined the impact sites of three more missiles in other locations, but there was insufficient evidence to calculate their trajectories.
HRW cautioned that the data was not "conclusive", but described it as "highly suggestive and another piece of the puzzle".
The UN team also identified the weapons used in the attack. They were two types of artillery-launched rocket: the M14 and the 330mm.
Both of these Russian-made projectiles are known to be in the arsenal of the regime's armed forces, which also possess the biggest stockpile of poison gas in the Middle East.
Russia insists that rebel forces carried out the gas attack, throwing into doubt the prospect of a rapid UN deal on Syrian disarmament.
The Foreign Minister, Sergei Lavrov, speaking after talks in Moscow with his French counterpart, Laurent Fabius, said that he had serious grounds to believe that the chemical attack was a provocation by rebels, despite France's insistence that the report shows Assad's regime was behind it.
Lavrov also again rejected Western demands that a UN Security Council resolution on chemical disarmament should include an automatic right to use force if Damascus fails to comply.
He said that Moscow would stick by its interpretation of the wording of the deal that he reached with the US in Geneva. Lavrov also said military force, under chapter seven of the UN charter, might be possible if Syria failed to surrender its chemical arsenal - but only after a second Security Council vote.
Western Governments fear that such a two-step agreement - although implied by the ambiguous wording of the Geneva deal - would allow Moscow to veto international backing for punitive strikes even if Syria drags its feet.
This would create domestic political embarrassment and legal difficulties for Washington and Paris if they went ahead with their suspended threat of punitive air strikes.
Damascus must submit a full inventory of its arsenal by the end of this week, admit international inspectors by November and surrender its chemical weapons by the middle of next year.
The Russians agreed to wording which threatens consequences if Syria fails to comply but - according to Moscow - stipulates that there must be a second UN resolution before any punitive action.