United States secretary of state John Kerry has laid the groundwork for possible military action against the Syrian government over a chemical weapons attack, implicating president Bashar al-Assad's forces in a "moral obscenity".
In the most forceful US reaction yet to an "undeniable" gas attack outside Damascus, Mr Kerry said president Barack Obama "believes there must be accountability for those who would use the world's most heinous weapons against the world's most vulnerable people".
Prime Minister Kevin Rudd it is "patently clear" there was a chemical weapons attack and he had spoken with Mr Obama, who expressed his grave concern and agreed to consult closely over possible options.
Mr Kerry's comments came after UN chemical weapons experts interviewed and took blood samples from victims of the attack, having themselves survived sniper fire that hit their convoy as they entered the zone.
"What we saw in Syria last week should shock the conscience of the world; it defies any code of morality," Mr Kerry said.
"Let me be clear: the indiscriminate slaughter of civilians, the killing of women and children and innocent bystanders by chemical weapons is a moral obscenity."
Mr Kerry accused the government of president Bashar al-Assad of acting like it had something to hide by blocking the UN inspectors' visit to the scene for days and shelling the area.
"Our sense of basic humanity is offended not only by this cowardly crime, but also by the cynical attempt to cover it up," Mr Kerry said.
He said Mr Obama will soon make an informed decision about how to respond to the indiscriminate and "obscene" attack.
The information so far, including videos and accounts from the ground, indicate chemical weapons were used in Syria, he said.
"It is undeniable," he said, adding that it was the Syrian government that maintained custody of chemical weapons and had the rockets capable of delivering them.
The indiscriminate slaughter of civilians, the killing of women and children and innocent bystanders by chemical weapons, is a moral obscenity.
Mr Kerry strongly implied that no-one else besides the Syrian regime could have been behind the attack and said the US had additional information it would provide in the days ahead.
Mr Rudd says he carefully noted Mr Kerry's remarks and agrees with his assessment.
"This is a grave breach of not just human rights, it is a humanitarian assault, and potentially also a crime against humanity," Mr Rudd told the ABC.
"For those reasons, the international community must now come together. We on the UN Security Council will do everything we can to forge a consensus."
Mr Rudd says Australia will have a significant role in responding to Syria because it takes over the presidency of the UN Security Council on Sunday.
Mr Assad has denied using chemical weapons and warned that any Western strikes against his regime would be doomed to failure.
Syria is understood to have the third largest stockpile of chemical weapons in the world - including sarin
and other nerve gases.
It is also one of the few countries not to have signed the Chemical Weapons Convention alongside Angola, Egypt, North Korea and South Sudan.
Amid accusations by Syrian activists that forces loyal to president Bashar al-Assad have used nerve gas to kill more than 200 people, we look back over similar allegations made during the conflict.
But there are mounting signs that the US and Western allies are laying the groundwork for some kind of military response, which took place a year after Mr Obama declared the use of chemical weapons a "red line" that would require strong action.
Mr Obama, who withdrew troops from Iraq and is winding down the conflict in Afghanistan, has been reluctant to intervene in two-and-a-half years of civil war in Syria.
A Reuters/Ipsos poll published on Saturday showed about 60 per cent of Americans opposed US military intervention, while only 9 per cent thought Mr Obama should act.
Antnony Cordesman from the US Centre for Strategic and International Studies says with one-fifth of Syria's population displaced by the civil war and at least 100,000 it is becoming more difficult for the US to stand by as the conflict continues.
"A third of the people in Syria are 14 years of age or younger, so you're talking a massive impact on children," he said.
"I think it's that cumulative pressure and the fact they've so clearly crossed the red for the second time, that's leading the administration to act."
US House of Representatives speaker John Boehner told the White House that it must consult with Congress before beginning any response.
"The speaker made clear that before any action is taken, there must be meaningful consultation with members of Congress, as well as clearly defined objectives and a broader strategy to achieve stability," spokesman Brendan Buck said in a statement.
The White House said national security adviser Susan Rice touched on Syria in a meeting with a delegation led by Yaakov Amidror, the chairman of Israel's National Security Council.
UN inspectors reach victims of chemical attack despite taking sniper fire
UN inspectors have spoken with victims of the suspected attack after their convoy was targeted by sniper fire which damaged one of their vehicles.
Syrian state media blamed rebels for the incident, but the opposition says pro-regime forces were trying to stop the inspectors from reaching the site.
UN secretary general Ban Ki Moon says he has instructed his envoy in Damascus to register a strong complaint with the Syrian government over the security lapse.
However, Mr Ban says, the inspectors had a productive visit to the alleged attack site.
"They visited two hospitals, they interviewed witnesses, survivors and doctors, they also collected some samples," he said.
"They have now returned to Damascus."
UN representative Martin Nesirky says the sniper attack was outrageous and both the government and opposition gave assurances the UN team would not be targeted.
"Now plainly that is something we expect all sides to live up to, so that this team of technical and medical experts can carry out the work that they've been sent there to do and are very keen to carry out," he said.
British PM cuts short holiday to tackle Syria
British prime minister David Cameron is cutting short his holiday to deal with the Syria situation, a Downing Street spokesman said.
Mr Cameron, who has been on holiday in Cornwall, south-west England, held a series of phone conversations with other world leaders over the weekend but headed to London to continue working on the international response.
Mr Cameron spoke with Russian president Vladimir Putin by telephone on Monday.
Mr Putin said "they did not have evidence of whether a chemical weapons attack had taken place or who was responsible", according to a Downing Street spokesman.
However, Mr Cameron doubted that the rebels had the capability to carry out such an attack and pointed out that the regime had launched a heavy offensive in the area in the days before and after the incident.
"The regime had also prevented UN access in the immediate aftermath, suggesting they had something to hide," he told Mr Putin.
The pair both repeated their commitment to an agreement reached by G8 leaders in June, which resolved that no-one should use chemical weapons and any use would merit a serious response from the international community.
Russia, also a veto-wielding member of the UN Security Council, has repeatedly warned Washington and its allies against military action in Syria.
Mr Cameron spoke with German chancellor Angela Merkel and French president Francois Hollande on Sunday, having spoken to Mr Obama and Canadian prime minister Stephen Harper on Saturday.
UK foreign secretary William Hague said an international response was possible without the formal backing of the UN.