The US Secretary of State John Kerry says the fight against the Islamic State is "a battle between civilization itself and barbarism".
Mr Kerry was speaking outside the French embassy in Paris where he met with French officials and embassy staff.
It comes as the hunt continues for those involved in terror attacks in the French capital last week.
"I have said many times, as have all of my predecessors, that France is America's oldest friend, our first ally. And during our darkest hours, that fact rings truer than ever" Mr Kerry said.
"Don't mistake what these attacks represent. This is not a clash of civilizations. These terrorists have declared war against all civilization".
"They rape and torture and pillage and call it the will of God. They are in fact psychopathic monsters, and there is nothing, nothing civilized about them".
"This is a battle between civilization itself and barbarism, between civilization and medieval and modern fascism both at the same time. And that is why every single nation-state in the region and around the world is opposed to Daesh" he added.
He was speaking before the embassy was lit up in the colours of the French flag - joining several other buildings around the world.
Tonight we lit up @USEmbassyFrance in bleu, blanc, & rouge. #USavecvous pic.twitter.com/qsXpv90XpD
— John Kerry (@JohnKerry) November 16, 2015
Secretary Kerry told reporters thier course - to defeat the Islamic State group - will not change.
French President Francois Hollande is meeting with Mr Kerry in Paris this morning.
