Iran’s chief nuclear negotiator said if Syria’s president was forced to stand down, the country would be left as a “haven for terrorists”.

Iran has pulled out of talks with six world powers in protest at the expansion of US sanctions, saying the blacklisting of more entities violates the spirit of a groundbreaking agreement over Tehran’s atomic programme.

US secretary of state John Kerry is flying to Geneva, Switzerland, to join six world powers for talks on Iran’s nuclear programmes, in a last-minute decision that suggests a deal could be imminent.