WASHINGTON: The United States said on Monday that chemical weapons had been used against Syrian civilians and warned President Barack Obama would demand accountability for this “moral obscenity.”

Employing his strongest language yet, US Secretary of State John Kerry said Washington was still examining evidence, but left no doubt that Bashar al-Assad's regime would be blamed.

“What we saw in Syria last week should shock the conscience of the world. It defies any code of morality,” he said, in a televised statement from the State Department.

“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,” he said.

“By any standard it is inexcusable, and despite the excuses and equivocations that some have manufactured, it is undeniable.”

Assad's government has denied carrying out last week's alleged chemical weapons strike on a civilian community near Damascus, which reportedly killed hundreds of people.

But Kerry said that independent reports of an atrocity were credible and said that the United States would soon present more concrete evidence of its own.

“Moreover, we know that the Syrian regime maintains custody of these chemical weapons. We know that the Syrian regime has the capacity to do this with rockets,” he said.

“We know that the regime has been determined to clear the opposition from those very places where the attacks took place. And with our own eyes, we have all of us become witnesses.

“We have additional information about this attack, and that information is being compiled and reviewed together with our partners, and we will provide that information in the days ahead,” he said.

“Our sense of basic humanity is offended not only by this cowardly crime, but also by the cynical attempt to cover it up.”

And, amid reports that US and allied forces are preparing to launch cruise missile strikes against the Syrian regime, Kerry warned that the chemical attack would have consequences.

“Make no mistake. President Obama believes there must be accountability for those who would use the world's most heinous weapons against the world's most vulnerable people. Nothing today is more serious, and nothing is receiving more serious scrutiny.”