Secretary of State John Kerry insisted Friday the United States is not alone in its quest to punish Bashar al-Assad's Syrian regime, citing the support of France, America's "oldest ally."

On Thursday, key US ally Britain dropped out of the coalition planning military strikes in response to Assad's use of chemical weapons following a parliamentary vote.

But Kerry said other countries supported the US push, notably Australia, members of the Organization of Islamic Cooperation and France, which famously refused to join the US-led war in Iraq but has said it is ready to join strikes on Syria.

"America should feel gratified. We are not alone in our condemnation an we're not alone in our will to do something about it and to act. The world is speaking out and many friends stand ready to respond," Kerry said.

"The Organization of Islamic Cooperation condemned the regime and said we needed, quote, to hold the Syrian government legally and morally accountable for this crime," he said.

"Our oldest ally, the French, said the regime committed this vile action and it's an outrage to use weapons that the community has banned for the last 90 years in all international conventions.

"The Australian prime minister said he didn't want history to report that we were party to turning such a blind eye."

Kerry's comments came in a speech in which he defended the apparently imminent military intervention as part of America's historic "duty" to prevent the use and spread of chemical weapons.