Secretary of State John Kerry said the US will channel $60 million in aid to Syrian groups the US chooses, which is a clear message that jihadi groups must be excluded from the aid.
Although the US aid is "non lethal", this is the biggest package and strongest back-up yet given so far by the US to Syrian rebels.
Mr Kerry said "no people should live in fear of their so called leaders" and "we must meet the moment."
US backing of the Syria rebels today is at its most public, financial, open and morally clear.
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The chancellor now faces the prospect of forming a minority government or calling for a snap election.
The 73-year-old said a return is unlikely "given the politicised atmosphere that seems to have afflicted our set."
Chancellor Philip Hammond said the UK will make a financial proposal to the EU in the next three-and-a-half weeks.