The United States Secretary of State John Kerry said there was “undeniable” evidence that a large-scale chemical weapons attack had been carried out on civilians in the Damascus suburbs, in Syria.

Kerry spoke to reporters at the State Department on Monday calling the attacks a “moral obscenity” that "should shock the conscience" of the world.

Officials say there is little doubt that this attack, which killed hundreds, including women and children, was the work of the Syrian government, headed by President Bashar al-Assad. 

However he has denied launching an attack. On Monday the United Nations expert investigators came under sniper fire as they traveled to the site of the August 21 attack.

The Secretary of State said, “The indiscriminate slaughter of civilians, the killing of women and children and innocent bystanders by chemical weapons is a moral obscenity. By any standard, it is inexcusable and — despite the excuses and equivocations that some have manufactured — it is undeniable," said Kerry, the highest-ranking U.S. official to confirm the attack in the Damascus suburbs that activists say killed hundreds of people.”

He added, "This international norm cannot be violated without consequences.”

Following a meeting between envoys and the Syrian National Coalition in Istanbul on Tuesday, Western powers have told the Syrian opposition to expect a strike against al-Assad within days, Reuters reports.

A source said, “The opposition was told in clear terms that action to deter further use of chemical weapons by the Assad regime could come as early as in the next few days, and that they should still prepare for peace talks at Geneva.”

Senior figures of the Syrian National Coalition were present at the meeting, including its president, Ahmad Jarba, and envoys from 11 core "Friends of Syria" alliance members, including Robert Ford, the former U.S. ambassador to Syria.

One source said that Ford told Jarba that the coalition should "expect appropriate action to deter more use of chemical weapons."

The Friends of Syria comprises the United States, Britain, France, Italy, Germany, Turkey, Saudi Arabia, Egypt, Jordan, the United Arab Emirates, and Qatar.

It has not been decided how this alliance will “strike” however White House spokesman Jay Carney said, “There is not a military solution to that conflict.”

However the AP reports that “The U.S., along with allies in Europe, appeared to be laying the groundwork for the most aggressive response since Syria's civil war began more than two years ago.”

Here’s a BBC interview with US Defence Secretary Chuck Hagel, who says the US military is "ready to go":