BEIRUT, Lebanon — All evidence in an independent investigation of the horrific chemical weapons attack in Damascus on Aug. 21 points to the Syrian government, according to a report released Tuesday by Human Rights Watch.

The 22-page report examines eyewitness accounts, medical records and physical remnants of the weapons, concluding that the type of rockets and launchers used in these attacks are “weapon systems known and documented to be only in the possession of, and used by, Syrian government armed forces.”

“We are certain this was a chemical weapon attack. We are certain it was a large-scale attack and we are almost certain the Syrian government is responsible,” Peter Bouckaert, emergencies director at Human Rights Watch and author of the report, said via phone Tuesday morning from Geneva. “The various theories advanced to suggest that opposition forces may have been responsible for the attack lack credibility.”

A US senate vote on President Barack Obama’s proposal to carry out strikes against the Syrian regime in response to the chemical attack was delayed Monday, as Russia and Syria showed willingness to negotiate a handover of Syria’s chemical stockpiles to international monitors to avoid US action.

“If the establishment of international control of chemical weapons in the country will help avoid military strikes, we will immediately start working with Damascus,” Russian Foreign Minister Sergey Lavrov said Monday. “We call on the Syrian leadership not only to agree to put chemical weapons storages under international control, but to also to have them destroyed subsequently.”

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But the Syrian regime continues to deny any involvement in the Aug. 21 attacks, claiming opposition forces are responsible.

“I wouldn’t put any weight to the government denial of this attack,” Bouckaert said. “They have also denied using cluster weapons, ballistic missiles, incendiary bombs — every crime committed in Syria has been denied by the regime. We need to deal with the facts on the ground rather than the denial of the Syrian government.”

Graphic by Human Rights Watch.

HRW identified the first of two types of missiles used in last month's attack as a 330mm rocket designed to deliver between 50 and 60 liters of liquid chemical agent. The second, a 140mm Soviet-made rocket, can be used in conjunction with three kinds of warheads, one of which delivers 2.2 kilograms of sarin chemical liquid.

The loading of such warheads requires a dangerous and specialized procedure. While government forces possess the technology and launchers required to load and fire these devices, there is no evidence to suggest that opposition groups do.

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Graphic by Human Rights Watch.

The missiles landed 16 kilometers (just under 10 miles) apart in the rebel-held suburbs of East and West Ghouta in Damascus. Medical facilities in the surrounding areas recorded 734 deaths in East Ghouta and 125 named victims in West Ghouta, with thousands more suffering severe reactions from exposure to the chemical.

Bouckaert said he believes the true number of fatalities to be much higher, as there was no comprehensive recording system in place within the overwhelmed medical facilities. In addition many of the victims were buried directly, their deaths going unregistered.

The missiles were fired from outside of the area, most likely Damascus military airport. An analysis of video evidence and interviews with doctors and victims points to sarin exposure, according to the report.

Syria is one of only five United Nations member states that have not signed the 1993 Chemical Weapons Convention that prohibits the development, production, stockpiling, and use of chemical agents.

The Aug. 21 attacks are the first major use of chemical weapons since the 1988 attack on the Kurdish city of Halabja by the Iraqi government of Saddam Hussein.