damascus gas attack evidence
UN chemical weapons experts escorted by Free Syrian Army fighters drive through Ghouta, Damascus (Reuters)

With the US about to present its intelligence report into the Syrian regime's responsibilities over the nerve gas massacre, details are starting to emerge that suggest that the attack on Ghouta was made on the direct orders of Bashar al-Assad to avenge an assassination bid.

The Office of the Director of National Intelligence have drawn up a timeline of the events that allegedly led to the death by poison gas of hundreds of people in the Damascus suburb of Ghouta, the Washington Post reported.

The timeline is likely to start from early August when a convoy carrying President Bashar al-Assad came under rebel fire in central Damascus.

The Syrian president escaped unscathed but the attack exposed the vulnerability of the regime and led to demands among senior military officers for a stern response.

Whitehall and West European sources suggested that chemical weapons were used against rebel-held area on Assad's direct orders out of revenge for the assasination attempt, The Times reported.

A diluted nerve agent designed "to teach rebels a lesson" was released on Ghouta during an attack led by the 4th Armoured Division, the regime's military backbone under the command of Assad's brother Maher, official sources said.

But the Ghouta attack reportedly surprised high-ranked government officials.

The US report contains intelligence wiretappings of frantic phone calls between an official at the Syrian Ministry of Defence and a leader of a chemical weapons unit discussing the chemical strike, foreign policy blog The Cable said.

The defence official was reportedly heard demanding answers over the use of nerve agents. If the conversation is verified, it would shine a dim light over the regime's control of its own weapon stockpiles.

A similar conversation was reportedly intercepted by Israeli Defence Forces, German magazine Focus reported.

Israeli spy services also fed the CIA with intelligence showing that the regime moved chemical weapons in the Ghouta area ahead of the attack, Arab diplomatic sources told the Wall Street Journal.

Evidence is said to include also satellite images, eyewitness accounts and medical reports.

US vice-president Joe Biden said there was no doubt that the attack was launched by the Syrian regime and that Assad must be held accountable.

UN inspectors were in Ghouta investigating whether chemical weapons were used and their findings are yet to be disclosed.

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