New US Secretary of State John Kerry has attended a gathering of nations in Rome who support the Syria opposition.
Foreign Secretary William Hague joined Mr Kerry as they met leaders of the Friends of Syria group.
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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.