A United Nations weapons inspectors' report has confirmed the use of sarin gas in a poison gas attack in the suburbs of Damascus last month.

As expected, the report does not say who launched the attack in the rebel-held suburb of Ghouta on August 21.

"The conclusion is that chemical weapons have been used in the ongoing conflict between the parties in Syria, also against civilians, including children, on a relatively large scale," the report by chief UN investigator Ake Sellstrom said.

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"In particular, the environmental, chemical and medical samples we have collected provide clear and convincing evidence that surface-to-surface rockets containing the nerve agent sarin were used."

Key points: UN report into August 21 attack confirms sarin gas was used

UN report into August 21 attack confirms sarin gas was used Report does not confirm who launched the attack

Report does not confirm who launched the attack Ban Ki Moon condemns the attack and says it is a war crime

Ban Ki Moon condemns the attack and says it is a war crime Most significant use of chemical weapons since Saddam Hussein

Most significant use of chemical weapons since Saddam Hussein US, France and UK welcome the report and maintain Assad is responsible

US, France and UK welcome the report and maintain Assad is responsible Rebels unhappy with weapons deal

Rebels unhappy with weapons deal Turkey claims its warplanes shot down a Syrian helicopter

UN secretary-general Ban Ki-moon has condemned the attack as a war crime, and called on the Security Council to impose "consequences" for any failure by president Bashar al-Assad to keep to a Russia-US plan to destroy Syria's banned chemical arsenal.

Mr Ban told the 15-nation council that UN investigators have "now confirmed, unequivocally and objectively that chemical weapons have been used in Syria".

"This is a war crime," he said.

"I trust all can join me in condemning this despicable crime.

"This is the most significant confirmed use of chemical weapons against civilians since Saddam Hussein used them in Halabja (Iraq) in 1988.

"The international community has pledged to prevent any such horror from recurring, yet it has happened again."

Mr Ban told the council how doctors treated civilians with no external signs of injuries in the streets of Ghouta in the hours after the attack.

"The weather conditions that morning were conducive to maximising the potential impact," Mr Ban said.

"The downward movement of air would have allowed the gas to easily penetrate the basements and lower levels of buildings and other structures where many people were seeking shelter."

The United States, which has threatened a military strike over the chemical arms, estimates 1,400 people died in the attack.

It believes the Assad regime is responsible.

US and Europe welcome the report, insist Assad is responsible

The White House has said the UN's report bolsters the US argument that the Syrian government was responsible.

President Barack Obama's national security adviser, Susan Rice, said in a statement that the technical evidence in the UN report, including that the sarin nerve agent was high-quality and that a particular rocket was used in the attack, was significant.

Chemical weapons in Syria: A UN report has confirmed that sarin gas was used in an attack on August 21 which is believed to have killed 1,400 people. While the report does not attribute blame, the US, France and UK maintain that president Bashar al-Assad used the weapons against his own people. Mr Assad has blamed the rebels for the attack. Syria, understood to have the third largest stockpile of chemical weapons in the world, has agreed to surrender them under a new deal. We look back over allegations chemical weapons have been used during the on going conflict.

The conclusion, she said, "reinforces our assessment that these attacks were carried out by the Syrian regime, as only they had the capability to mount an attack in this manner."

The US ambassador to the UN, Samantha Power, highlighted the use of a 122mm rocket, which she said only government forces had.

She also pointed to the "high quality" sarin used in the attack.

Britain has also welcomed the report saying it was "abundantly clear" from the findings that Mr Assad's regime was to blame.

"We welcome this objective and unequivocal report which confirms that chemical weapons were used on a large scale," foreign secretary William Hague said in a statement.

"From the wealth of technical detail in the report... it is abundantly clear that the Syrian regime is the only party that could have been responsible."

Meanwhile, French foreign minister Laurent Fabius said there was "no doubt" government forces were to blame.

Assad must face consequences if he fails to comply with the UN

Earlier, Britain, France and the US launched a push for a tough UN resolution on Syria's chemical weapons and moved to bolster rebels fighting to oust Mr Assad.

Kick-starting a week of intense diplomatic activity in the wake of a weekend US-Russia deal on the dismantling of Syria's deadly arsenal, the three powers agreed in Paris talks that Mr Assad must face "serious consequences" if he fails to comply with a UN resolution implementing the accord.

President Bashar al-Assad has been given a week to hand over details of his chemical weapons stockpiles. ( AFP/HO/SANA )

The US-Russia deal gives Mr Assad a week to hand over details of his chemical weapons stockpiles, and calls for inspections, of what the US says are some 45 sites linked to the program, to be underway by November with an aim of neutralising the country's chemical capacity by mid-2014.

The Organisation for the Prohibition of Chemical Weapons (OPCW) said its experts "are already at work preparing a roadmap that anticipates the various challenges involved in verifying Syria's declared stockpiles".

"It is envisaged... that the program to eliminate chemical weapons in Syria will be initiated in a matter of days," the Hague-based OPCW said.

Mr Assad's regime is to provide "on an expeditious basis" a complete inventory of its chemical weapons, production facilities, and "related materials" to the OPCW.

Watchdog experts "will verify the accuracy of this disclosure with on-site inspections, and will also assist in putting into place arrangements to keep the warfare materials and the relevant facilities secure until their destruction."

Also, under the deal, Syria joins the Chemical Weapons Convention, which will come into force in Syria on October 14, the OPCW said.

The 41-member council is expected to meet on Thursday or Friday.

"I am aware of the onerous responsibility that the international community is placing on our shoulders," OPCW director-general Ahmet Uzumcu was quoted as saying.

"We will bring to bear on this mission our full energies and commitment, and I have every confidence that the international community will support us fully."

Meeting with rebel leaders on sidelines of UN meeting

The tough talking triggered an immediate warning from Russia that Western sabre-rattling could derail efforts to bring the regime and rebels to the table for negotiations aimed at ending a civil war that has raged for more than two years and left more than 110,000 people dead.

The deal was greeted with dismay by rebel leaders, who fear the West's willingness to do business with Mr Assad will consolidate his grip on power and stall the momentum of moves to provide them with the arms they need to tilt the balance of the civil war in their favour.

French foreign minister Laurent Fabius and US secretary of state John Kerry attempted to reassure the rebels they had not been forgotten, with Mr Fabius announcing an international meeting with leaders of the Syrian National Coalition on the sidelines of the UN General Assembly in New York next week.

"We know that in order to negotiate a political solution, there has to be a strong opposition," Mr Fabius said.

France has long championed the opposition coalition, but there is concern in other Western capitals about the prominent role that hardened Islamist fighters are playing in the fight against Mr Assad's forces.

Turkey claims to have shot down Syrian helicopter

Meanwhile, Turkey claims its warplanes shot down a Syrian helicopter after it crossed into Turkish airspace and the government warned it had taken all necessary measures to defend itself against any further such violations.

Turkey scrambled two F-16 jets along the border between its southern Hatay province and Syria, after warning the Mi-17 helicopter it was approaching Turkish airspace shortly before 8:30pm on Monday, the military said in a statement.

Deputy prime minister Bulent Arinc said a warplane shot down the helicopter after it ventured up to 2km into Turkey near the border town of Yayladagi.

"It was repeatedly warned by our air defence elements," he said.

It came down in a ball of flames inside Syrian territory after being hit, amateur video footage showed.

"Turkey will definitely not allow any violation of its borders... we will defend our borders and our people's security to the end," Turkish foreign minister Ahmet Davutoglu said.

"No one will have the nerve to violate Turkey's borders in any way again," he said.

He said details of the incident would be provided to the UN Security Council and fellow members of the NATO military alliance.

Turkey, one of Mr Assad's fiercest critics, has advocated military intervention in Syria and grown frustrated over what it considers to be Western indecisiveness.

It shares a 900km border with Syria and is sheltering a quarter of the 2 million people who have fled the conflict.

AFP