Britain has upgraded the status of the London office of the Syrian opposition to a mission, Foreign Secretary William Hague said on Thursday, as U.S. Secretary of State John Kerry described the Syrian presidential vote planned for June as an "insult" and a "farce."

The office of the Syrian National Coalition was being upgraded "in recognition of the strength of our partnership," Hague said after a meeting of the Friends of Syria group in London.

For his part, Kerry said in London: "Together we are agreed in saying that Assad's staged elections are a farce, they are an insult and they are a fraud."