US President Donald Trump has urged Iranians to continue nationwide protests and seize control of state institutions, saying on social media that he has cancelled all meetings with Iranian officials and warning that those responsible for killing protesters would “pay a big price,” while also announcing a 25 per cent tariff on countries doing business with Iran, as Tehran responded sharply with its security chief accusing Trump and Israeli Prime Minister Benjamin Netanyahu of being responsible for violence against the Iranian people, amid claims that diplomatic channels with Iran remain open.