Secretary of State John Kerry described Syrian President Assad a "thug and a murderer" and the world had a choice of what to do.
Mr Kerry told a press conference: "Some cite the risk of doing things, but we need to ask what is the risk of doing nothing? It matters because if we choose to live in a world where a thug and a murderer like Bashar al-Assad can gas thousands of his own people with impunity.
"Even after the US and our allies said no and then the world does nothing about it - There will be no end to the test of our resolve."
More top news
Weather throws a wobbly as it's seriously disappointing for summer.
The Bank of England governor has suggested there may soon be a cut in interest rates.
The motorist at the wheel of a self-driving car has died after it crashed with a trailer, the first Autopilot death reported in the US.