Syrian government forces were "almost certainly" responsible for the two chemical weapons attacks on opposition-controlled areas of Damascus last month, an investigation by Human Rights Watch has found.

Analysis of the type of rockets and launchers used in the August 21 attacks outside Damascus, as well as photos and videos from the attack sites, implicated Syrian government forces, HRW found. Interviews with survivors and medical workers, also supported this finding.

There was no evidence to suggest, as President Bashar al-Assad has said, that the opposition fighters were responsible for the attacks, the report noted.

The release of the findings comes as France announced it would take a resolution to the United Nations Security Council that aimed to bring Syria’s chemical weapons stockpile under international control.

Meanwhile, as the US pushes forward with its case for military intervention in Syria in response to the suspected sarin attacks, Russia presented its own plan to deal with Syria’s chemical weapons, proposing that the Syrian leadership place its weapons in "storage sites under international control", and agree on "their subsequent destruction and fully joining the treaty on the prohibition of chemical weapons".

The August 21 attacks killed hundreds of civilians – horrific videos shot in the aftermath show people convulsing, frothing at the mouth and suffering from dizziness, restricted and irregular breathing.

The missiles containing the nerve agents landed in two rebel-controlled neighbourhoods of East and West Ghouta in Damascus, Human Rights Watch said.

Although it did not pinpoint an exact death toll, the report noted that medical workers in East Ghouta recorded 734 deaths. In West Ghouta Human Rights Watch collected the names of at least 80 dead, while Medecins Sans Frontiers reported at least 3600 people were treated for symptoms consistent with exposure to neurotoxins.

At just over 800 the death toll is significantly lower than the figure of 1429 that is being used to justify President Barak Obama’s push for US military intervention in Syria.

"This was a sophisticated, planned military attack on two opposition-held areas 16 kilometres apart", Peter Bouckaert, emergencies director at Human Rights Watch and author of the report, told Fairfax Media.

"The sophistication and expertise needed to load these weapons is significant … we have never seen these systems in the hands of the opposition."

The HRW investigation found the attacks were conducted using a surface-to-surface rocket system of approximately 330mm in diameter – likely Syrian-produced – and a Soviet-era 140mm surface-to-surface rocket system to deliver a nerve agent.

"We can be definitive that this was a chemical weapons attack using a military grade chemical agent, most likely sarin," Mr Bouckaert said. "These are weapon systems known and documented to be only in the possession of, and used by, Syrian government armed forces."

HRW was not able to gain access to the scene of the attack but instead carried out interviews with survivors and medical staff, consulted chemical weapons experts and analysed images of rocket fragments taken by sources in Damascus.

It shared the evidence it collected with the United Nations chemical weapons investigators, who entered Syria in the days after the attack, collecting samples, interviewing witnesses and visiting hospitals. Their report is expected to be delivered in the coming days.

The UN report would provide "important, definitive scientific proof that this was a chemical attack," Mr Bouckaert said.

Yet one limitation of the UN investigation, is that it is mandated not to assign blame for the attack, and this, he said, is "the most important question facing and dividing the international community right now."

The United Nations estimates that more than 100,000 people have died since the conflict began in Syria in March 2011, while at least five million Syrians are internally displaced and another two million have fled, mostly to neighbouring countries such as Lebanon and Turkey.